Merge remote-tracking branch 'dhowells/netfs-lib'

Pick up David Howells' netfs helper library and the new fscache API.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Documentation/filesystems/index.rst

@@ -53,6 +53,7 @@ filesystem implementations.
    journalling
    fscrypt
    fsverity
+   netfs_library
 
 Filesystems
 ===========

Documentation/filesystems/netfs_library.rst

@@ -0,0 +1,526 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+NETWORK FILESYSTEM HELPER LIBRARY
+=================================
+
+.. Contents:
+
+ - Overview.
+ - Buffered read helpers.
+   - Read helper functions.
+   - Read helper structures.
+   - Read helper operations.
+   - Read helper procedure.
+   - Read helper cache API.
+
+
+Overview
+========
+
+The network filesystem helper library is a set of functions designed to aid a
+network filesystem in implementing VM/VFS operations.  For the moment, that
+just includes turning various VM buffered read operations into requests to read
+from the server.  The helper library, however, can also interpose other
+services, such as local caching or local data encryption.
+
+Note that the library module doesn't link against local caching directly, so
+access must be provided by the netfs.
+
+
+Buffered Read Helpers
+=====================
+
+The library provides a set of read helpers that handle the ->readpage(),
+->readahead() and much of the ->write_begin() VM operations and translate them
+into a common call framework.
+
+The following services are provided:
+
+ * Handles transparent huge pages (THPs).
+
+ * Insulates the netfs from VM interface changes.
+
+ * Allows the netfs to arbitrarily split reads up into pieces, even ones that
+   don't match page sizes or page alignments and that may cross pages.
+
+ * Allows the netfs to expand a readahead request in both directions to meet
+   its needs.
+
+ * Allows the netfs to partially fulfil a read, which will then be resubmitted.
+
+ * Handles local caching, allowing cached data and server-read data to be
+   interleaved for a single request.
+
+ * Handles clearing of bufferage that aren't on the server.
+
+ * Handle retrying of reads that failed, switching reads from the cache to the
+   server as necessary.
+
+ * In the future, this is a place that other services can be performed, such as
+   local encryption of data to be stored remotely or in the cache.
+
+From the network filesystem, the helpers require a table of operations.  This
+includes a mandatory method to issue a read operation along with a number of
+optional methods.
+
+
+Read Helper Functions
+---------------------
+
+Three read helpers are provided::
+
+ * void netfs_readahead(struct readahead_control *ractl,
+			const struct netfs_read_request_ops *ops,
+			void *netfs_priv);``
+ * int netfs_readpage(struct file *file,
+		      struct page *page,
+		      const struct netfs_read_request_ops *ops,
+		      void *netfs_priv);
+ * int netfs_write_begin(struct file *file,
+			 struct address_space *mapping,
+			 loff_t pos,
+			 unsigned int len,
+			 unsigned int flags,
+			 struct page **_page,
+			 void **_fsdata,
+			 const struct netfs_read_request_ops *ops,
+			 void *netfs_priv);
+
+Each corresponds to a VM operation, with the addition of a couple of parameters
+for the use of the read helpers:
+
+ * ``ops``
+
+   A table of operations through which the helpers can talk to the filesystem.
+
+ * ``netfs_priv``
+
+   Filesystem private data (can be NULL).
+
+Both of these values will be stored into the read request structure.
+
+For ->readahead() and ->readpage(), the network filesystem should just jump
+into the corresponding read helper; whereas for ->write_begin(), it may be a
+little more complicated as the network filesystem might want to flush
+conflicting writes or track dirty data and needs to put the acquired page if an
+error occurs after calling the helper.
+
+The helpers manage the read request, calling back into the network filesystem
+through the suppplied table of operations.  Waits will be performed as
+necessary before returning for helpers that are meant to be synchronous.
+
+If an error occurs and netfs_priv is non-NULL, ops->cleanup() will be called to
+deal with it.  If some parts of the request are in progress when an error
+occurs, the request will get partially completed if sufficient data is read.
+
+Additionally, there is::
+
+  * void netfs_subreq_terminated(struct netfs_read_subrequest *subreq,
+				 ssize_t transferred_or_error,
+				 bool was_async);
+
+which should be called to complete a read subrequest.  This is given the number
+of bytes transferred or a negative error code, plus a flag indicating whether
+the operation was asynchronous (ie. whether the follow-on processing can be
+done in the current context, given this may involve sleeping).
+
+
+Read Helper Structures
+----------------------
+
+The read helpers make use of a couple of structures to maintain the state of
+the read.  The first is a structure that manages a read request as a whole::
+
+	struct netfs_read_request {
+		struct inode		*inode;
+		struct address_space	*mapping;
+		struct netfs_cache_resources cache_resources;
+		void			*netfs_priv;
+		loff_t			start;
+		size_t			len;
+		loff_t			i_size;
+		const struct netfs_read_request_ops *netfs_ops;
+		unsigned int		debug_id;
+		...
+	};
+
+The above fields are the ones the netfs can use.  They are:
+
+ * ``inode``
+ * ``mapping``
+
+   The inode and the address space of the file being read from.  The mapping
+   may or may not point to inode->i_data.
+
+ * ``cache_resources``
+
+   Resources for the local cache to use, if present.
+
+ * ``netfs_priv``
+
+   The network filesystem's private data.  The value for this can be passed in
+   to the helper functions or set during the request.  The ->cleanup() op will
+   be called if this is non-NULL at the end.
+
+ * ``start``
+ * ``len``
+
+   The file position of the start of the read request and the length.  These
+   may be altered by the ->expand_readahead() op.
+
+ * ``i_size``
+
+   The size of the file at the start of the request.
+
+ * ``netfs_ops``
+
+   A pointer to the operation table.  The value for this is passed into the
+   helper functions.
+
+ * ``debug_id``
+
+   A number allocated to this operation that can be displayed in trace lines
+   for reference.
+
+
+The second structure is used to manage individual slices of the overall read
+request::
+
+	struct netfs_read_subrequest {
+		struct netfs_read_request *rreq;
+		loff_t			start;
+		size_t			len;
+		size_t			transferred;
+		unsigned long		flags;
+		unsigned short		debug_index;
+		...
+	};
+
+Each subrequest is expected to access a single source, though the helpers will
+handle falling back from one source type to another.  The members are:
+
+ * ``rreq``
+
+   A pointer to the read request.
+
+ * ``start``
+ * ``len``
+
+   The file position of the start of this slice of the read request and the
+   length.
+
+ * ``transferred``
+
+   The amount of data transferred so far of the length of this slice.  The
+   network filesystem or cache should start the operation this far into the
+   slice.  If a short read occurs, the helpers will call again, having updated
+   this to reflect the amount read so far.
+
+ * ``flags``
+
+   Flags pertaining to the read.  There are two of interest to the filesystem
+   or cache:
+
+   * ``NETFS_SREQ_CLEAR_TAIL``
+
+     This can be set to indicate that the remainder of the slice, from
+     transferred to len, should be cleared.
+
+   * ``NETFS_SREQ_SEEK_DATA_READ``
+
+     This is a hint to the cache that it might want to try skipping ahead to
+     the next data (ie. using SEEK_DATA).
+
+ * ``debug_index``
+
+   A number allocated to this slice that can be displayed in trace lines for
+   reference.
+
+
+Read Helper Operations
+----------------------
+
+The network filesystem must provide the read helpers with a table of operations
+through which it can issue requests and negotiate::
+
+	struct netfs_read_request_ops {
+		void (*init_rreq)(struct netfs_read_request *rreq, struct file *file);
+		bool (*is_cache_enabled)(struct inode *inode);
+		int (*begin_cache_operation)(struct netfs_read_request *rreq);
+		void (*expand_readahead)(struct netfs_read_request *rreq);
+		bool (*clamp_length)(struct netfs_read_subrequest *subreq);
+		void (*issue_op)(struct netfs_read_subrequest *subreq);
+		bool (*is_still_valid)(struct netfs_read_request *rreq);
+		int (*check_write_begin)(struct file *file, loff_t pos, unsigned len,
+					 struct page *page, void **_fsdata);
+		void (*done)(struct netfs_read_request *rreq);
+		void (*cleanup)(struct address_space *mapping, void *netfs_priv);
+	};
+
+The operations are as follows:
+
+ * ``init_rreq()``
+
+   [Optional] This is called to initialise the request structure.  It is given
+   the file for reference and can modify the ->netfs_priv value.
+
+ * ``is_cache_enabled()``
+
+   [Required] This is called by netfs_write_begin() to ask if the file is being
+   cached.  It should return true if it is being cached and false otherwise.
+
+ * ``begin_cache_operation()``
+
+   [Optional] This is called to ask the network filesystem to call into the
+   cache (if present) to initialise the caching state for this read.  The netfs
+   library module cannot access the cache directly, so the cache should call
+   something like fscache_begin_read_operation() to do this.
+
+   The cache gets to store its state in ->cache_resources and must set a table
+   of operations of its own there (though of a different type).
+
+   This should return 0 on success and an error code otherwise.  If an error is
+   reported, the operation may proceed anyway, just without local caching (only
+   out of memory and interruption errors cause failure here).
+
+ * ``expand_readahead()``
+
+   [Optional] This is called to allow the filesystem to expand the size of a
+   readahead read request.  The filesystem gets to expand the request in both
+   directions, though it's not permitted to reduce it as the numbers may
+   represent an allocation already made.  If local caching is enabled, it gets
+   to expand the request first.
+
+   Expansion is communicated by changing ->start and ->len in the request
+   structure.  Note that if any change is made, ->len must be increased by at
+   least as much as ->start is reduced.
+
+ * ``clamp_length()``
+
+   [Optional] This is called to allow the filesystem to reduce the size of a
+   subrequest.  The filesystem can use this, for example, to chop up a request
+   that has to be split across multiple servers or to put multiple reads in
+   flight.
+
+   This should return 0 on success and an error code on error.
+
+ * ``issue_op()``
+
+   [Required] The helpers use this to dispatch a subrequest to the server for
+   reading.  In the subrequest, ->start, ->len and ->transferred indicate what
+   data should be read from the server.
+
+   There is no return value; the netfs_subreq_terminated() function should be
+   called to indicate whether or not the operation succeeded and how much data
+   it transferred.  The filesystem also should not deal with setting pages
+   uptodate, unlocking them or dropping their refs - the helpers need to deal
+   with this as they have to coordinate with copying to the local cache.
+
+   Note that the helpers have the pages locked, but not pinned.  It is possible
+   to use the ITER_XARRAY iov iterator to refer to the range of the inode that
+   is being operated upon without the need to allocate large bvec tables.
+
+ * ``is_still_valid()``
+
+   [Optional] This is called to find out if the data just read from the local
+   cache is still valid.  It should return true if it is still valid and false
+   if not.  If it's not still valid, it will be reread from the server.
+
+ * ``check_write_begin()``
+
+   [Optional] This is called from the netfs_write_begin() helper once it has
+   allocated/grabbed the page to be modified to allow the filesystem to flush
+   conflicting state before allowing it to be modified.
+
+   It should return 0 if everything is now fine, -EAGAIN if the page should be
+   regrabbed and any other error code to abort the operation.
+
+ * ``done``
+
+   [Optional] This is called after the pages in the request have all been
+   unlocked (and marked uptodate if applicable).
+
+ * ``cleanup``
+
+   [Optional] This is called as the request is being deallocated so that the
+   filesystem can clean up ->netfs_priv.
+
+
+
+Read Helper Procedure
+---------------------
+
+The read helpers work by the following general procedure:
+
+ * Set up the request.
+
+ * For readahead, allow the local cache and then the network filesystem to
+   propose expansions to the read request.  This is then proposed to the VM.
+   If the VM cannot fully perform the expansion, a partially expanded read will
+   be performed, though this may not get written to the cache in its entirety.
+
+ * Loop around slicing chunks off of the request to form subrequests:
+
+   * If a local cache is present, it gets to do the slicing, otherwise the
+     helpers just try to generate maximal slices.
+
+   * The network filesystem gets to clamp the size of each slice if it is to be
+     the source.  This allows rsize and chunking to be implemented.
+
+   * The helpers issue a read from the cache or a read from the server or just
+     clears the slice as appropriate.
+
+   * The next slice begins at the end of the last one.
+
+   * As slices finish being read, they terminate.
+
+ * When all the subrequests have terminated, the subrequests are assessed and
+   any that are short or have failed are reissued:
+
+   * Failed cache requests are issued against the server instead.
+
+   * Failed server requests just fail.
+
+   * Short reads against either source will be reissued against that source
+     provided they have transferred some more data:
+
+     * The cache may need to skip holes that it can't do DIO from.
+
+     * If NETFS_SREQ_CLEAR_TAIL was set, a short read will be cleared to the
+       end of the slice instead of reissuing.
+
+ * Once the data is read, the pages that have been fully read/cleared:
+
+   * Will be marked uptodate.
+
+   * If a cache is present, will be marked with PG_fscache.
+
+   * Unlocked
+
+ * Any pages that need writing to the cache will then have DIO writes issued.
+
+ * Synchronous operations will wait for reading to be complete.
+
+ * Writes to the cache will proceed asynchronously and the pages will have the
+   PG_fscache mark removed when that completes.
+
+ * The request structures will be cleaned up when everything has completed.
+
+
+Read Helper Cache API
+---------------------
+
+When implementing a local cache to be used by the read helpers, two things are
+required: some way for the network filesystem to initialise the caching for a
+read request and a table of operations for the helpers to call.
+
+The network filesystem's ->begin_cache_operation() method is called to set up a
+cache and this must call into the cache to do the work.  If using fscache, for
+example, the cache would call::
+
+	int fscache_begin_read_operation(struct netfs_read_request *rreq,
+					 struct fscache_cookie *cookie);
+
+passing in the request pointer and the cookie corresponding to the file.
+
+The netfs_read_request object contains a place for the cache to hang its
+state::
+
+	struct netfs_cache_resources {
+		const struct netfs_cache_ops	*ops;
+		void				*cache_priv;
+		void				*cache_priv2;
+	};
+
+This contains an operations table pointer and two private pointers.  The
+operation table looks like the following::
+
+	struct netfs_cache_ops {
+		void (*end_operation)(struct netfs_cache_resources *cres);
+
+		void (*expand_readahead)(struct netfs_cache_resources *cres,
+					 loff_t *_start, size_t *_len, loff_t i_size);
+
+		enum netfs_read_source (*prepare_read)(struct netfs_read_subrequest *subreq,
+						       loff_t i_size);
+
+		int (*read)(struct netfs_cache_resources *cres,
+			    loff_t start_pos,
+			    struct iov_iter *iter,
+			    bool seek_data,
+			    netfs_io_terminated_t term_func,
+			    void *term_func_priv);
+
+		int (*write)(struct netfs_cache_resources *cres,
+			     loff_t start_pos,
+			     struct iov_iter *iter,
+			     netfs_io_terminated_t term_func,
+			     void *term_func_priv);
+	};
+
+With a termination handler function pointer::
+
+	typedef void (*netfs_io_terminated_t)(void *priv,
+					      ssize_t transferred_or_error,
+					      bool was_async);
+
+The methods defined in the table are:
+
+ * ``end_operation()``
+
+   [Required] Called to clean up the resources at the end of the read request.
+
+ * ``expand_readahead()``
+
+   [Optional] Called at the beginning of a netfs_readahead() operation to allow
+   the cache to expand a request in either direction.  This allows the cache to
+   size the request appropriately for the cache granularity.
+
+   The function is passed poiners to the start and length in its parameters,
+   plus the size of the file for reference, and adjusts the start and length
+   appropriately.  It should return one of:
+
+   * ``NETFS_FILL_WITH_ZEROES``
+   * ``NETFS_DOWNLOAD_FROM_SERVER``
+   * ``NETFS_READ_FROM_CACHE``
+   * ``NETFS_INVALID_READ``
+
+   to indicate whether the slice should just be cleared or whether it should be
+   downloaded from the server or read from the cache - or whether slicing
+   should be given up at the current point.
+
+ * ``prepare_read()``
+
+   [Required] Called to configure the next slice of a request.  ->start and
+   ->len in the subrequest indicate where and how big the next slice can be;
+   the cache gets to reduce the length to match its granularity requirements.
+
+ * ``read()``
+
+   [Required] Called to read from the cache.  The start file offset is given
+   along with an iterator to read to, which gives the length also.  It can be
+   given a hint requesting that it seek forward from that start position for
+   data.
+
+   Also provided is a pointer to a termination handler function and private
+   data to pass to that function.  The termination function should be called
+   with the number of bytes transferred or an error code, plus a flag
+   indicating whether the termination is definitely happening in the caller's
+   context.
+
+ * ``write()``
+
+   [Required] Called to write to the cache.  The start file offset is given
+   along with an iterator to write from, which gives the length also.
+
+   Also provided is a pointer to a termination handler function and private
+   data to pass to that function.  The termination function should be called
+   with the number of bytes transferred or an error code, plus a flag
+   indicating whether the termination is definitely happening in the caller's
+   context.
+
+Note that these methods are passed a pointer to the cache resource structure,
+not the read request structure as they could be used in other situations where
+there isn't a read request structure as well, such as writing dirty data to the
+cache.

fs/Kconfig

@@ -125,6 +125,7 @@ source "fs/overlayfs/Kconfig"
 
 menu "Caches"
 
+source "fs/netfs/Kconfig"
 source "fs/fscache/Kconfig"
 source "fs/cachefiles/Kconfig"
 

fs/Makefile

@@ -67,6 +67,7 @@ obj-y				+= devpts/
 obj-$(CONFIG_DLM)		+= dlm/
  
 # Do not add any filesystems before this line
+obj-$(CONFIG_NETFS_SUPPORT)	+= netfs/
 obj-$(CONFIG_FSCACHE)		+= fscache/
 obj-$(CONFIG_REISERFS_FS)	+= reiserfs/
 obj-$(CONFIG_EXT4_FS)		+= ext4/

fs/cachefiles/Makefile

@@ -7,6 +7,7 @@ cachefiles-y := \
 	bind.o \
 	daemon.o \
 	interface.o \
+	io.o \
 	key.o \
 	main.o \
 	namei.o \

fs/cachefiles/interface.c

@@ -319,8 +319,8 @@ static void cachefiles_drop_object(struct fscache_object *_object)
 /*
  * dispose of a reference to an object
  */
-static void cachefiles_put_object(struct fscache_object *_object,
-				  enum fscache_obj_ref_trace why)
+void cachefiles_put_object(struct fscache_object *_object,
+			   enum fscache_obj_ref_trace why)
 {
 	struct cachefiles_object *object;
 	struct fscache_cache *cache;
@@ -568,4 +568,5 @@ const struct fscache_cache_ops cachefiles_cache_ops = {
 	.uncache_page		= cachefiles_uncache_page,
 	.dissociate_pages	= cachefiles_dissociate_pages,
 	.check_consistency	= cachefiles_check_consistency,
+	.begin_read_operation	= cachefiles_begin_read_operation,
 };

fs/cachefiles/internal.h

@@ -150,6 +150,9 @@ extern int cachefiles_has_space(struct cachefiles_cache *cache,
  */
 extern const struct fscache_cache_ops cachefiles_cache_ops;
 
+void cachefiles_put_object(struct fscache_object *_object,
+			   enum fscache_obj_ref_trace why);
+
 /*
  * key.c
  */
@@ -217,6 +220,12 @@ extern int cachefiles_allocate_pages(struct fscache_retrieval *,
 extern int cachefiles_write_page(struct fscache_storage *, struct page *);
 extern void cachefiles_uncache_page(struct fscache_object *, struct page *);
 
+/*
+ * rdwr2.c
+ */
+extern int cachefiles_begin_read_operation(struct netfs_read_request *,
+					   struct fscache_retrieval *);
+
 /*
  * security.c
  */

fs/cachefiles/io.c

@@ -0,0 +1,420 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* kiocb-using read/write
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/mount.h>
+#include <linux/slab.h>
+#include <linux/file.h>
+#include <linux/uio.h>
+#include <linux/sched/mm.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+struct cachefiles_kiocb {
+	struct kiocb		iocb;
+	refcount_t		ki_refcnt;
+	loff_t			start;
+	union {
+		size_t		skipped;
+		size_t		len;
+	};
+	netfs_io_terminated_t	term_func;
+	void			*term_func_priv;
+	bool			was_async;
+};
+
+static inline void cachefiles_put_kiocb(struct cachefiles_kiocb *ki)
+{
+	if (refcount_dec_and_test(&ki->ki_refcnt)) {
+		fput(ki->iocb.ki_filp);
+		kfree(ki);
+	}
+}
+
+/*
+ * Handle completion of a read from the cache.
+ */
+static void cachefiles_read_complete(struct kiocb *iocb, long ret, long ret2)
+{
+	struct cachefiles_kiocb *ki = container_of(iocb, struct cachefiles_kiocb, iocb);
+
+	_enter("%ld,%ld", ret, ret2);
+
+	if (ki->term_func) {
+		if (ret >= 0)
+			ret += ki->skipped;
+		ki->term_func(ki->term_func_priv, ret, ki->was_async);
+	}
+
+	cachefiles_put_kiocb(ki);
+}
+
+/*
+ * Initiate a read from the cache.
+ */
+static int cachefiles_read(struct netfs_cache_resources *cres,
+			   loff_t start_pos,
+			   struct iov_iter *iter,
+			   bool seek_data,
+			   netfs_io_terminated_t term_func,
+			   void *term_func_priv)
+{
+	struct cachefiles_kiocb *ki;
+	struct file *file = cres->cache_priv2;
+	unsigned int old_nofs;
+	ssize_t ret = -ENOBUFS;
+	size_t len = iov_iter_count(iter), skipped = 0;
+
+	_enter("%pD,%li,%llx,%zx/%llx",
+	       file, file_inode(file)->i_ino, start_pos, len,
+	       i_size_read(file->f_inode));
+
+	/* If the caller asked us to seek for data before doing the read, then
+	 * we should do that now.  If we find a gap, we fill it with zeros.
+	 */
+	if (seek_data) {
+		loff_t off = start_pos, off2;
+
+		off2 = vfs_llseek(file, off, SEEK_DATA);
+		if (off2 < 0 && off2 >= (loff_t)-MAX_ERRNO && off2 != -ENXIO) {
+			skipped = 0;
+			ret = off2;
+			goto presubmission_error;
+		}
+
+		if (off2 == -ENXIO || off2 >= start_pos + len) {
+			/* The region is beyond the EOF or there's no more data
+			 * in the region, so clear the rest of the buffer and
+			 * return success.
+			 */
+			iov_iter_zero(len, iter);
+			skipped = len;
+			ret = 0;
+			goto presubmission_error;
+		}
+
+		skipped = off2 - off;
+		iov_iter_zero(skipped, iter);
+	}
+
+	ret = -ENOBUFS;
+	ki = kzalloc(sizeof(struct cachefiles_kiocb), GFP_KERNEL);
+	if (!ki)
+		goto presubmission_error;
+
+	refcount_set(&ki->ki_refcnt, 2);
+	ki->iocb.ki_filp	= file;
+	ki->iocb.ki_pos		= start_pos + skipped;
+	ki->iocb.ki_flags	= IOCB_DIRECT;
+	ki->iocb.ki_hint	= ki_hint_validate(file_write_hint(file));
+	ki->iocb.ki_ioprio	= get_current_ioprio();
+	ki->skipped		= skipped;
+	ki->term_func		= term_func;
+	ki->term_func_priv	= term_func_priv;
+	ki->was_async		= true;
+
+	if (ki->term_func)
+		ki->iocb.ki_complete = cachefiles_read_complete;
+
+	get_file(ki->iocb.ki_filp);
+
+	old_nofs = memalloc_nofs_save();
+	ret = vfs_iocb_iter_read(file, &ki->iocb, iter);
+	memalloc_nofs_restore(old_nofs);
+	switch (ret) {
+	case -EIOCBQUEUED:
+		goto in_progress;
+
+	case -ERESTARTSYS:
+	case -ERESTARTNOINTR:
+	case -ERESTARTNOHAND:
+	case -ERESTART_RESTARTBLOCK:
+		/* There's no easy way to restart the syscall since other AIO's
+		 * may be already running. Just fail this IO with EINTR.
+		 */
+		ret = -EINTR;
+		fallthrough;
+	default:
+		ki->was_async = false;
+		cachefiles_read_complete(&ki->iocb, ret, 0);
+		if (ret > 0)
+			ret = 0;
+		break;
+	}
+
+in_progress:
+	cachefiles_put_kiocb(ki);
+	_leave(" = %zd", ret);
+	return ret;
+
+presubmission_error:
+	if (term_func)
+		term_func(term_func_priv, ret < 0 ? ret : skipped, false);
+	return ret;
+}
+
+/*
+ * Handle completion of a write to the cache.
+ */
+static void cachefiles_write_complete(struct kiocb *iocb, long ret, long ret2)
+{
+	struct cachefiles_kiocb *ki = container_of(iocb, struct cachefiles_kiocb, iocb);
+	struct inode *inode = file_inode(ki->iocb.ki_filp);
+
+	_enter("%ld,%ld", ret, ret2);
+
+	/* Tell lockdep we inherited freeze protection from submission thread */
+	__sb_writers_acquired(inode->i_sb, SB_FREEZE_WRITE);
+	__sb_end_write(inode->i_sb, SB_FREEZE_WRITE);
+
+	if (ki->term_func)
+		ki->term_func(ki->term_func_priv, ret, ki->was_async);
+
+	cachefiles_put_kiocb(ki);
+}
+
+/*
+ * Initiate a write to the cache.
+ */
+static int cachefiles_write(struct netfs_cache_resources *cres,
+			    loff_t start_pos,
+			    struct iov_iter *iter,
+			    netfs_io_terminated_t term_func,
+			    void *term_func_priv)
+{
+	struct cachefiles_kiocb *ki;
+	struct inode *inode;
+	struct file *file = cres->cache_priv2;
+	unsigned int old_nofs;
+	ssize_t ret = -ENOBUFS;
+	size_t len = iov_iter_count(iter);
+
+	_enter("%pD,%li,%llx,%zx/%llx",
+	       file, file_inode(file)->i_ino, start_pos, len,
+	       i_size_read(file->f_inode));
+
+	ki = kzalloc(sizeof(struct cachefiles_kiocb), GFP_KERNEL);
+	if (!ki)
+		goto presubmission_error;
+
+	refcount_set(&ki->ki_refcnt, 2);
+	ki->iocb.ki_filp	= file;
+	ki->iocb.ki_pos		= start_pos;
+	ki->iocb.ki_flags	= IOCB_DIRECT | IOCB_WRITE;
+	ki->iocb.ki_hint	= ki_hint_validate(file_write_hint(file));
+	ki->iocb.ki_ioprio	= get_current_ioprio();
+	ki->start		= start_pos;
+	ki->len			= len;
+	ki->term_func		= term_func;
+	ki->term_func_priv	= term_func_priv;
+	ki->was_async		= true;
+
+	if (ki->term_func)
+		ki->iocb.ki_complete = cachefiles_write_complete;
+
+	/* Open-code file_start_write here to grab freeze protection, which
+	 * will be released by another thread in aio_complete_rw().  Fool
+	 * lockdep by telling it the lock got released so that it doesn't
+	 * complain about the held lock when we return to userspace.
+	 */
+	inode = file_inode(file);
+	__sb_start_write(inode->i_sb, SB_FREEZE_WRITE);
+	__sb_writers_release(inode->i_sb, SB_FREEZE_WRITE);
+
+	get_file(ki->iocb.ki_filp);
+
+	old_nofs = memalloc_nofs_save();
+	ret = vfs_iocb_iter_write(file, &ki->iocb, iter);
+	memalloc_nofs_restore(old_nofs);
+	switch (ret) {
+	case -EIOCBQUEUED:
+		goto in_progress;
+
+	case -ERESTARTSYS:
+	case -ERESTARTNOINTR:
+	case -ERESTARTNOHAND:
+	case -ERESTART_RESTARTBLOCK:
+		/* There's no easy way to restart the syscall since other AIO's
+		 * may be already running. Just fail this IO with EINTR.
+		 */
+		ret = -EINTR;
+		fallthrough;
+	default:
+		ki->was_async = false;
+		cachefiles_write_complete(&ki->iocb, ret, 0);
+		if (ret > 0)
+			ret = 0;
+		break;
+	}
+
+in_progress:
+	cachefiles_put_kiocb(ki);
+	_leave(" = %zd", ret);
+	return ret;
+
+presubmission_error:
+	if (term_func)
+		term_func(term_func_priv, -ENOMEM, false);
+	return -ENOMEM;
+}
+
+/*
+ * Prepare a read operation, shortening it to a cached/uncached
+ * boundary as appropriate.
+ */
+static enum netfs_read_source cachefiles_prepare_read(struct netfs_read_subrequest *subreq,
+						      loff_t i_size)
+{
+	struct fscache_retrieval *op = subreq->rreq->cache_resources.cache_priv;
+	struct cachefiles_object *object;
+	struct cachefiles_cache *cache;
+	const struct cred *saved_cred;
+	struct file *file = subreq->rreq->cache_resources.cache_priv2;
+	loff_t off, to;
+
+	_enter("%zx @%llx/%llx", subreq->len, subreq->start, i_size);
+
+	object = container_of(op->op.object,
+			      struct cachefiles_object, fscache);
+	cache = container_of(object->fscache.cache,
+			     struct cachefiles_cache, cache);
+
+	if (!file)
+		goto cache_fail_nosec;
+
+	if (subreq->start >= i_size)
+		return NETFS_FILL_WITH_ZEROES;
+
+	cachefiles_begin_secure(cache, &saved_cred);
+
+	off = vfs_llseek(file, subreq->start, SEEK_DATA);
+	if (off < 0 && off >= (loff_t)-MAX_ERRNO) {
+		if (off == (loff_t)-ENXIO)
+			goto download_and_store;
+		goto cache_fail;
+	}
+
+	if (off >= subreq->start + subreq->len)
+		goto download_and_store;
+
+	if (off > subreq->start) {
+		off = round_up(off, cache->bsize);
+		subreq->len = off - subreq->start;
+		goto download_and_store;
+	}
+
+	to = vfs_llseek(file, subreq->start, SEEK_HOLE);
+	if (to < 0 && to >= (loff_t)-MAX_ERRNO)
+		goto cache_fail;
+
+	if (to < subreq->start + subreq->len) {
+		if (subreq->start + subreq->len >= i_size)
+			to = round_up(to, cache->bsize);
+		else
+			to = round_down(to, cache->bsize);
+		subreq->len = to - subreq->start;
+	}
+
+	cachefiles_end_secure(cache, saved_cred);
+	return NETFS_READ_FROM_CACHE;
+
+download_and_store:
+	if (cachefiles_has_space(cache, 0, (subreq->len + PAGE_SIZE - 1) / PAGE_SIZE) == 0)
+		__set_bit(NETFS_SREQ_WRITE_TO_CACHE, &subreq->flags);
+cache_fail:
+	cachefiles_end_secure(cache, saved_cred);
+cache_fail_nosec:
+	return NETFS_DOWNLOAD_FROM_SERVER;
+}
+
+/*
+ * Prepare for a write to occur.
+ */
+static int cachefiles_prepare_write(struct netfs_cache_resources *cres,
+				    loff_t *_start, size_t *_len, loff_t i_size)
+{
+	loff_t start = *_start;
+	size_t len = *_len, down;
+
+	/* Round to DIO size */
+	down = start - round_down(start, PAGE_SIZE);
+	*_start = start - down;
+	*_len = round_up(down + len, PAGE_SIZE);
+	return 0;
+}
+
+/*
+ * Clean up an operation.
+ */
+static void cachefiles_end_operation(struct netfs_cache_resources *cres)
+{
+	struct fscache_retrieval *op = cres->cache_priv;
+	struct file *file = cres->cache_priv2;
+
+	_enter("");
+
+	if (file)
+		fput(file);
+	if (op) {
+		fscache_op_complete(&op->op, false);
+		fscache_put_retrieval(op);
+	}
+
+	_leave("");
+}
+
+static const struct netfs_cache_ops cachefiles_netfs_cache_ops = {
+	.end_operation		= cachefiles_end_operation,
+	.read			= cachefiles_read,
+	.write			= cachefiles_write,
+	.prepare_read		= cachefiles_prepare_read,
+	.prepare_write		= cachefiles_prepare_write,
+};
+
+/*
+ * Open the cache file when beginning a cache operation.
+ */
+int cachefiles_begin_read_operation(struct netfs_read_request *rreq,
+				    struct fscache_retrieval *op)
+{
+	struct cachefiles_object *object;
+	struct cachefiles_cache *cache;
+	struct path path;
+	struct file *file;
+
+	_enter("");
+
+	object = container_of(op->op.object,
+			      struct cachefiles_object, fscache);
+	cache = container_of(object->fscache.cache,
+			     struct cachefiles_cache, cache);
+
+	path.mnt = cache->mnt;
+	path.dentry = object->backer;
+	file = open_with_fake_path(&path, O_RDWR | O_LARGEFILE | O_DIRECT,
+				   d_inode(object->backer), cache->cache_cred);
+	if (IS_ERR(file))
+		return PTR_ERR(file);
+	if (!S_ISREG(file_inode(file)->i_mode))
+		goto error_file;
+	if (unlikely(!file->f_op->read_iter) ||
+	    unlikely(!file->f_op->write_iter)) {
+		pr_notice("Cache does not support read_iter and write_iter\n");
+		goto error_file;
+	}
+
+	fscache_get_retrieval(op);
+	rreq->cache_resources.cache_priv = op;
+	rreq->cache_resources.cache_priv2 = file;
+	rreq->cache_resources.ops = &cachefiles_netfs_cache_ops;
+	rreq->cookie_debug_id = object->fscache.debug_id;
+	_leave("");
+	return 0;
+
+error_file:
+	fput(file);
+	return -EIO;
+}

fs/ext4/verity.c

@@ -370,7 +370,7 @@ static struct page *ext4_read_merkle_tree_page(struct inode *inode,
 					       pgoff_t index,
 					       unsigned long num_ra_pages)
 {
-	DEFINE_READAHEAD(ractl, NULL, inode->i_mapping, index);
+	DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
 	struct page *page;
 
 	index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;

fs/f2fs/file.c

@@ -4051,7 +4051,7 @@ out:
 
 static int redirty_blocks(struct inode *inode, pgoff_t page_idx, int len)
 {
-	DEFINE_READAHEAD(ractl, NULL, inode->i_mapping, page_idx);
+	DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, page_idx);
 	struct address_space *mapping = inode->i_mapping;
 	struct page *page;
 	pgoff_t redirty_idx = page_idx;

fs/f2fs/verity.c

@@ -228,7 +228,7 @@ static struct page *f2fs_read_merkle_tree_page(struct inode *inode,
 					       pgoff_t index,
 					       unsigned long num_ra_pages)
 {
-	DEFINE_READAHEAD(ractl, NULL, inode->i_mapping, index);
+	DEFINE_READAHEAD(ractl, NULL, NULL, inode->i_mapping, index);
 	struct page *page;
 
 	index += f2fs_verity_metadata_pos(inode) >> PAGE_SHIFT;

fs/fscache/Kconfig

@@ -2,6 +2,7 @@
 
 config FSCACHE
 	tristate "General filesystem local caching manager"
+	select NETFS_SUPPORT
 	help
 	  This option enables a generic filesystem caching manager that can be
 	  used by various network and other filesystems to cache data locally.

fs/fscache/Makefile

@@ -7,6 +7,7 @@ fscache-y := \
 	cache.o \
 	cookie.o \
 	fsdef.o \
+	io.o \
 	main.o \
 	netfs.o \
 	object.o \

fs/fscache/internal.h

@@ -142,6 +142,10 @@ extern int fscache_wait_for_operation_activation(struct fscache_object *,
 						 atomic_t *,
 						 atomic_t *);
 extern void fscache_invalidate_writes(struct fscache_cookie *);
+struct fscache_retrieval *fscache_alloc_retrieval(struct fscache_cookie *cookie,
+						  struct address_space *mapping,
+						  fscache_rw_complete_t end_io_func,
+						  void *context);
 
 /*
  * proc.c

fs/fscache/io.c

@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Cache data I/O routines
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#define FSCACHE_DEBUG_LEVEL PAGE
+#include <linux/module.h>
+#define FSCACHE_USE_NEW_IO_API
+#include <linux/fscache-cache.h>
+#include <linux/slab.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+/*
+ * Start a cache read operation.
+ * - we return:
+ *   -ENOMEM	- out of memory, some pages may be being read
+ *   -ERESTARTSYS - interrupted, some pages may be being read
+ *   -ENOBUFS	- no backing object or space available in which to cache any
+ *                pages not being read
+ *   -ENODATA	- no data available in the backing object for some or all of
+ *                the pages
+ *   0		- dispatched a read on all pages
+ */
+int __fscache_begin_read_operation(struct netfs_read_request *rreq,
+				   struct fscache_cookie *cookie)
+{
+	struct fscache_retrieval *op;
+	struct fscache_object *object;
+	bool wake_cookie = false;
+	int ret;
+
+	_enter("rr=%08x", rreq->debug_id);
+
+	fscache_stat(&fscache_n_retrievals);
+
+	if (hlist_empty(&cookie->backing_objects))
+		goto nobufs;
+
+	if (test_bit(FSCACHE_COOKIE_INVALIDATING, &cookie->flags)) {
+		_leave(" = -ENOBUFS [invalidating]");
+		return -ENOBUFS;
+	}
+
+	ASSERTCMP(cookie->def->type, !=, FSCACHE_COOKIE_TYPE_INDEX);
+
+	if (fscache_wait_for_deferred_lookup(cookie) < 0)
+		return -ERESTARTSYS;
+
+	op = fscache_alloc_retrieval(cookie, NULL, NULL, NULL);
+	if (!op)
+		return -ENOMEM;
+	trace_fscache_page_op(cookie, NULL, &op->op, fscache_page_op_retr_multi);
+
+	spin_lock(&cookie->lock);
+
+	if (!fscache_cookie_enabled(cookie) ||
+	    hlist_empty(&cookie->backing_objects))
+		goto nobufs_unlock;
+	object = hlist_entry(cookie->backing_objects.first,
+			     struct fscache_object, cookie_link);
+
+	__fscache_use_cookie(cookie);
+	atomic_inc(&object->n_reads);
+	__set_bit(FSCACHE_OP_DEC_READ_CNT, &op->op.flags);
+
+	if (fscache_submit_op(object, &op->op) < 0)
+		goto nobufs_unlock_dec;
+	spin_unlock(&cookie->lock);
+
+	fscache_stat(&fscache_n_retrieval_ops);
+
+	/* we wait for the operation to become active, and then process it
+	 * *here*, in this thread, and not in the thread pool */
+	ret = fscache_wait_for_operation_activation(
+		object, &op->op,
+		__fscache_stat(&fscache_n_retrieval_op_waits),
+		__fscache_stat(&fscache_n_retrievals_object_dead));
+	if (ret < 0)
+		goto error;
+
+	/* ask the cache to honour the operation */
+	ret = object->cache->ops->begin_read_operation(rreq, op);
+
+error:
+	if (ret == -ENOMEM)
+		fscache_stat(&fscache_n_retrievals_nomem);
+	else if (ret == -ERESTARTSYS)
+		fscache_stat(&fscache_n_retrievals_intr);
+	else if (ret == -ENODATA)
+		fscache_stat(&fscache_n_retrievals_nodata);
+	else if (ret < 0)
+		fscache_stat(&fscache_n_retrievals_nobufs);
+	else
+		fscache_stat(&fscache_n_retrievals_ok);
+
+	fscache_put_retrieval(op);
+	_leave(" = %d", ret);
+	return ret;
+
+nobufs_unlock_dec:
+	atomic_dec(&object->n_reads);
+	wake_cookie = __fscache_unuse_cookie(cookie);
+nobufs_unlock:
+	spin_unlock(&cookie->lock);
+	fscache_put_retrieval(op);
+	if (wake_cookie)
+		__fscache_wake_unused_cookie(cookie);
+nobufs:
+	fscache_stat(&fscache_n_retrievals_nobufs);
+	_leave(" = -ENOBUFS");
+	return -ENOBUFS;
+}
+EXPORT_SYMBOL(__fscache_begin_read_operation);

fs/fscache/page.c

@@ -299,7 +299,7 @@ static void fscache_release_retrieval_op(struct fscache_operation *_op)
 /*
  * allocate a retrieval op
  */
-static struct fscache_retrieval *fscache_alloc_retrieval(
+struct fscache_retrieval *fscache_alloc_retrieval(
 	struct fscache_cookie *cookie,
 	struct address_space *mapping,
 	fscache_rw_complete_t end_io_func,

fs/fscache/stats.c

@@ -278,5 +278,6 @@ int fscache_stats_show(struct seq_file *m, void *v)
 		   atomic_read(&fscache_n_cache_stale_objects),
 		   atomic_read(&fscache_n_cache_retired_objects),
 		   atomic_read(&fscache_n_cache_culled_objects));
+	netfs_stats_show(m);
 	return 0;
 }

fs/netfs/Kconfig

@@ -0,0 +1,23 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config NETFS_SUPPORT
+	tristate "Support for network filesystem high-level I/O"
+	help
+	  This option enables support for network filesystems, including
+	  helpers for high-level buffered I/O, abstracting out read
+	  segmentation, local caching and transparent huge page support.
+
+config NETFS_STATS
+	bool "Gather statistical information on local caching"
+	depends on NETFS_SUPPORT && PROC_FS
+	help
+	  This option causes statistical information to be gathered on local
+	  caching and exported through file:
+
+		/proc/fs/fscache/stats
+
+	  The gathering of statistics adds a certain amount of overhead to
+	  execution as there are a quite a few stats gathered, and on a
+	  multi-CPU system these may be on cachelines that keep bouncing
+	  between CPUs.  On the other hand, the stats are very useful for
+	  debugging purposes.  Saying 'Y' here is recommended.

fs/netfs/Makefile

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0
+
+netfs-y := read_helper.o stats.o
+
+obj-$(CONFIG_NETFS_SUPPORT) := netfs.o

fs/netfs/internal.h

@@ -0,0 +1,97 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Internal definitions for network filesystem support
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#ifdef pr_fmt
+#undef pr_fmt
+#endif
+
+#define pr_fmt(fmt) "netfs: " fmt
+
+/*
+ * read_helper.c
+ */
+extern unsigned int netfs_debug;
+
+/*
+ * stats.c
+ */
+#ifdef CONFIG_NETFS_STATS
+extern atomic_t netfs_n_rh_readahead;
+extern atomic_t netfs_n_rh_readpage;
+extern atomic_t netfs_n_rh_rreq;
+extern atomic_t netfs_n_rh_sreq;
+extern atomic_t netfs_n_rh_download;
+extern atomic_t netfs_n_rh_download_done;
+extern atomic_t netfs_n_rh_download_failed;
+extern atomic_t netfs_n_rh_download_instead;
+extern atomic_t netfs_n_rh_read;
+extern atomic_t netfs_n_rh_read_done;
+extern atomic_t netfs_n_rh_read_failed;
+extern atomic_t netfs_n_rh_zero;
+extern atomic_t netfs_n_rh_short_read;
+extern atomic_t netfs_n_rh_write;
+extern atomic_t netfs_n_rh_write_begin;
+extern atomic_t netfs_n_rh_write_done;
+extern atomic_t netfs_n_rh_write_failed;
+extern atomic_t netfs_n_rh_write_zskip;
+
+
+static inline void netfs_stat(atomic_t *stat)
+{
+	atomic_inc(stat);
+}
+
+static inline void netfs_stat_d(atomic_t *stat)
+{
+	atomic_dec(stat);
+}
+
+#else
+#define netfs_stat(x) do {} while(0)
+#define netfs_stat_d(x) do {} while(0)
+#endif
+
+/*****************************************************************************/
+/*
+ * debug tracing
+ */
+#define dbgprintk(FMT, ...) \
+	printk("[%-6.6s] "FMT"\n", current->comm, ##__VA_ARGS__)
+
+#define kenter(FMT, ...) dbgprintk("==> %s("FMT")", __func__, ##__VA_ARGS__)
+#define kleave(FMT, ...) dbgprintk("<== %s()"FMT"", __func__, ##__VA_ARGS__)
+#define kdebug(FMT, ...) dbgprintk(FMT, ##__VA_ARGS__)
+
+#ifdef __KDEBUG
+#define _enter(FMT, ...) kenter(FMT, ##__VA_ARGS__)
+#define _leave(FMT, ...) kleave(FMT, ##__VA_ARGS__)
+#define _debug(FMT, ...) kdebug(FMT, ##__VA_ARGS__)
+
+#elif defined(CONFIG_NETFS_DEBUG)
+#define _enter(FMT, ...)			\
+do {						\
+	if (netfs_debug)			\
+		kenter(FMT, ##__VA_ARGS__);	\
+} while (0)
+
+#define _leave(FMT, ...)			\
+do {						\
+	if (netfs_debug)			\
+		kleave(FMT, ##__VA_ARGS__);	\
+} while (0)
+
+#define _debug(FMT, ...)			\
+do {						\
+	if (netfs_debug)			\
+		kdebug(FMT, ##__VA_ARGS__);	\
+} while (0)
+
+#else
+#define _enter(FMT, ...) no_printk("==> %s("FMT")", __func__, ##__VA_ARGS__)
+#define _leave(FMT, ...) no_printk("<== %s()"FMT"", __func__, ##__VA_ARGS__)
+#define _debug(FMT, ...) no_printk(FMT, ##__VA_ARGS__)
+#endif

fs/netfs/stats.c

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Netfs support statistics
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/export.h>
+#include <linux/seq_file.h>
+#include <linux/netfs.h>
+#include "internal.h"
+
+atomic_t netfs_n_rh_readahead;
+atomic_t netfs_n_rh_readpage;
+atomic_t netfs_n_rh_rreq;
+atomic_t netfs_n_rh_sreq;
+atomic_t netfs_n_rh_download;
+atomic_t netfs_n_rh_download_done;
+atomic_t netfs_n_rh_download_failed;
+atomic_t netfs_n_rh_download_instead;
+atomic_t netfs_n_rh_read;
+atomic_t netfs_n_rh_read_done;
+atomic_t netfs_n_rh_read_failed;
+atomic_t netfs_n_rh_zero;
+atomic_t netfs_n_rh_short_read;
+atomic_t netfs_n_rh_write;
+atomic_t netfs_n_rh_write_begin;
+atomic_t netfs_n_rh_write_done;
+atomic_t netfs_n_rh_write_failed;
+atomic_t netfs_n_rh_write_zskip;
+
+void netfs_stats_show(struct seq_file *m)
+{
+	seq_printf(m, "RdHelp : RA=%u RP=%u WB=%u WBZ=%u rr=%u sr=%u\n",
+		   atomic_read(&netfs_n_rh_readahead),
+		   atomic_read(&netfs_n_rh_readpage),
+		   atomic_read(&netfs_n_rh_write_begin),
+		   atomic_read(&netfs_n_rh_write_zskip),
+		   atomic_read(&netfs_n_rh_rreq),
+		   atomic_read(&netfs_n_rh_sreq));
+	seq_printf(m, "RdHelp : ZR=%u sh=%u sk=%u\n",
+		   atomic_read(&netfs_n_rh_zero),
+		   atomic_read(&netfs_n_rh_short_read),
+		   atomic_read(&netfs_n_rh_write_zskip));
+	seq_printf(m, "RdHelp : DL=%u ds=%u df=%u di=%u\n",
+		   atomic_read(&netfs_n_rh_download),
+		   atomic_read(&netfs_n_rh_download_done),
+		   atomic_read(&netfs_n_rh_download_failed),
+		   atomic_read(&netfs_n_rh_download_instead));
+	seq_printf(m, "RdHelp : RD=%u rs=%u rf=%u\n",
+		   atomic_read(&netfs_n_rh_read),
+		   atomic_read(&netfs_n_rh_read_done),
+		   atomic_read(&netfs_n_rh_read_failed));
+	seq_printf(m, "RdHelp : WR=%u ws=%u wf=%u\n",
+		   atomic_read(&netfs_n_rh_write),
+		   atomic_read(&netfs_n_rh_write_done),
+		   atomic_read(&netfs_n_rh_write_failed));
+}
+EXPORT_SYMBOL(netfs_stats_show);

include/linux/fs.h

@@ -891,18 +891,22 @@ struct fown_struct {
 	int signum;		/* posix.1b rt signal to be delivered on IO */
 };
 
-/*
- * Track a single file's readahead state
+/**
+ * struct file_ra_state - Track a file's readahead state.
+ * @start: Where the most recent readahead started.
+ * @size: Number of pages read in the most recent readahead.
+ * @async_size: Start next readahead when this many pages are left.
+ * @ra_pages: Maximum size of a readahead request.
+ * @mmap_miss: How many mmap accesses missed in the page cache.
+ * @prev_pos: The last byte in the most recent read request.
  */
 struct file_ra_state {
-	pgoff_t start;			/* where readahead started */
-	unsigned int size;		/* # of readahead pages */
-	unsigned int async_size;	/* do asynchronous readahead when
-					   there are only # of pages ahead */
-
-	unsigned int ra_pages;		/* Maximum readahead window */
-	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
-	loff_t prev_pos;		/* Cache last read() position */
+	pgoff_t start;
+	unsigned int size;
+	unsigned int async_size;
+	unsigned int ra_pages;
+	unsigned int mmap_miss;
+	loff_t prev_pos;
 };
 
 /*

include/linux/fscache-cache.h

@@ -304,6 +304,10 @@ struct fscache_cache_ops {
 
 	/* dissociate a cache from all the pages it was backing */
 	void (*dissociate_pages)(struct fscache_cache *cache);
+
+	/* Begin a read operation for the netfs lib */
+	int (*begin_read_operation)(struct netfs_read_request *rreq,
+				    struct fscache_retrieval *op);
 };
 
 extern struct fscache_cookie fscache_fsdef_index;

include/linux/fscache.h

@@ -19,6 +19,7 @@
 #include <linux/pagemap.h>
 #include <linux/pagevec.h>
 #include <linux/list_bl.h>
+#include <linux/netfs.h>
 
 #if defined(CONFIG_FSCACHE) || defined(CONFIG_FSCACHE_MODULE)
 #define fscache_available() (1)
@@ -29,16 +30,6 @@
 #endif
 
 
-/*
- * overload PG_private_2 to give us PG_fscache - this is used to indicate that
- * a page is currently backed by a local disk cache
- */
-#define PageFsCache(page)		PagePrivate2((page))
-#define SetPageFsCache(page)		SetPagePrivate2((page))
-#define ClearPageFsCache(page)		ClearPagePrivate2((page))
-#define TestSetPageFsCache(page)	TestSetPagePrivate2((page))
-#define TestClearPageFsCache(page)	TestClearPagePrivate2((page))
-
 /* pattern used to fill dead space in an index entry */
 #define FSCACHE_INDEX_DEADFILL_PATTERN 0x79
 
@@ -46,6 +37,7 @@ struct pagevec;
 struct fscache_cache_tag;
 struct fscache_cookie;
 struct fscache_netfs;
+struct netfs_read_request;
 
 typedef void (*fscache_rw_complete_t)(struct page *page,
 				      void *context,
@@ -200,6 +192,10 @@ extern void __fscache_update_cookie(struct fscache_cookie *, const void *);
 extern int __fscache_attr_changed(struct fscache_cookie *);
 extern void __fscache_invalidate(struct fscache_cookie *);
 extern void __fscache_wait_on_invalidate(struct fscache_cookie *);
+
+#ifdef FSCACHE_USE_NEW_IO_API
+extern int __fscache_begin_read_operation(struct netfs_read_request *, struct fscache_cookie *);
+#else
 extern int __fscache_read_or_alloc_page(struct fscache_cookie *,
 					struct page *,
 					fscache_rw_complete_t,
@@ -223,6 +219,8 @@ extern void __fscache_uncache_all_inode_pages(struct fscache_cookie *,
 					      struct inode *);
 extern void __fscache_readpages_cancel(struct fscache_cookie *cookie,
 				       struct list_head *pages);
+#endif /* FSCACHE_USE_NEW_IO_API */
+
 extern void __fscache_disable_cookie(struct fscache_cookie *, const void *, bool);
 extern void __fscache_enable_cookie(struct fscache_cookie *, const void *, loff_t,
 				    bool (*)(void *), void *);
@@ -507,6 +505,36 @@ int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size)
 	return -ENOBUFS;
 }
 
+#ifdef FSCACHE_USE_NEW_IO_API
+
+/**
+ * fscache_begin_read_operation - Begin a read operation for the netfs lib
+ * @rreq: The read request being undertaken
+ * @cookie: The cookie representing the cache object
+ *
+ * Begin a read operation on behalf of the netfs helper library.  @rreq
+ * indicates the read request to which the operation state should be attached;
+ * @cookie indicates the cache object that will be accessed.
+ *
+ * This is intended to be called from the ->begin_cache_operation() netfs lib
+ * operation as implemented by the network filesystem.
+ *
+ * Returns:
+ * * 0		- Success
+ * * -ENOBUFS	- No caching available
+ * * Other error code from the cache, such as -ENOMEM.
+ */
+static inline
+int fscache_begin_read_operation(struct netfs_read_request *rreq,
+				 struct fscache_cookie *cookie)
+{
+	if (fscache_cookie_valid(cookie) && fscache_cookie_enabled(cookie))
+		return __fscache_begin_read_operation(rreq, cookie);
+	return -ENOBUFS;
+}
+
+#else /* FSCACHE_USE_NEW_IO_API */
+
 /**
  * fscache_read_or_alloc_page - Read a page from the cache or allocate a block
  * in which to store it
@@ -786,6 +814,8 @@ void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie,
 		__fscache_uncache_all_inode_pages(cookie, inode);
 }
 
+#endif /* FSCACHE_USE_NEW_IO_API */
+
 /**
  * fscache_disable_cookie - Disable a cookie
  * @cookie: The cookie representing the cache object

include/linux/netfs.h

@@ -0,0 +1,234 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Network filesystem support services.
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ *
+ * See:
+ *
+ *	Documentation/filesystems/netfs_library.rst
+ *
+ * for a description of the network filesystem interface declared here.
+ */
+
+#ifndef _LINUX_NETFS_H
+#define _LINUX_NETFS_H
+
+#include <linux/workqueue.h>
+#include <linux/fs.h>
+#include <linux/pagemap.h>
+
+/*
+ * Overload PG_private_2 to give us PG_fscache - this is used to indicate that
+ * a page is currently backed by a local disk cache
+ */
+#define PageFsCache(page)		PagePrivate2((page))
+#define SetPageFsCache(page)		SetPagePrivate2((page))
+#define ClearPageFsCache(page)		ClearPagePrivate2((page))
+#define TestSetPageFsCache(page)	TestSetPagePrivate2((page))
+#define TestClearPageFsCache(page)	TestClearPagePrivate2((page))
+
+/**
+ * set_page_fscache - Set PG_fscache on a page and take a ref
+ * @page: The page.
+ *
+ * Set the PG_fscache (PG_private_2) flag on a page and take the reference
+ * needed for the VM to handle its lifetime correctly.  This sets the flag and
+ * takes the reference unconditionally, so care must be taken not to set the
+ * flag again if it's already set.
+ */
+static inline void set_page_fscache(struct page *page)
+{
+	set_page_private_2(page);
+}
+
+/**
+ * end_page_fscache - Clear PG_fscache and release any waiters
+ * @page: The page
+ *
+ * Clear the PG_fscache (PG_private_2) bit on a page and wake up any sleepers
+ * waiting for this.  The page ref held for PG_private_2 being set is released.
+ *
+ * This is, for example, used when a netfs page is being written to a local
+ * disk cache, thereby allowing writes to the cache for the same page to be
+ * serialised.
+ */
+static inline void end_page_fscache(struct page *page)
+{
+	end_page_private_2(page);
+}
+
+/**
+ * wait_on_page_fscache - Wait for PG_fscache to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_fscache (aka PG_private_2) to be cleared on a page.
+ */
+static inline void wait_on_page_fscache(struct page *page)
+{
+	wait_on_page_private_2(page);
+}
+
+/**
+ * wait_on_page_fscache_killable - Wait for PG_fscache to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_fscache (aka PG_private_2) to be cleared on a page or until a
+ * fatal signal is received by the calling task.
+ *
+ * Return:
+ * - 0 if successful.
+ * - -EINTR if a fatal signal was encountered.
+ */
+static inline int wait_on_page_fscache_killable(struct page *page)
+{
+	return wait_on_page_private_2_killable(page);
+}
+
+enum netfs_read_source {
+	NETFS_FILL_WITH_ZEROES,
+	NETFS_DOWNLOAD_FROM_SERVER,
+	NETFS_READ_FROM_CACHE,
+	NETFS_INVALID_READ,
+} __mode(byte);
+
+typedef void (*netfs_io_terminated_t)(void *priv, ssize_t transferred_or_error,
+				      bool was_async);
+
+/*
+ * Resources required to do operations on a cache.
+ */
+struct netfs_cache_resources {
+	const struct netfs_cache_ops	*ops;
+	void				*cache_priv;
+	void				*cache_priv2;
+};
+
+/*
+ * Descriptor for a single component subrequest.
+ */
+struct netfs_read_subrequest {
+	struct netfs_read_request *rreq;	/* Supervising read request */
+	struct list_head	rreq_link;	/* Link in rreq->subrequests */
+	loff_t			start;		/* Where to start the I/O */
+	size_t			len;		/* Size of the I/O */
+	size_t			transferred;	/* Amount of data transferred */
+	refcount_t		usage;
+	short			error;		/* 0 or error that occurred */
+	unsigned short		debug_index;	/* Index in list (for debugging output) */
+	enum netfs_read_source	source;		/* Where to read from */
+	unsigned long		flags;
+#define NETFS_SREQ_WRITE_TO_CACHE	0	/* Set if should write to cache */
+#define NETFS_SREQ_CLEAR_TAIL		1	/* Set if the rest of the read should be cleared */
+#define NETFS_SREQ_SHORT_READ		2	/* Set if there was a short read from the cache */
+#define NETFS_SREQ_SEEK_DATA_READ	3	/* Set if ->read() should SEEK_DATA first */
+#define NETFS_SREQ_NO_PROGRESS		4	/* Set if we didn't manage to read any data */
+};
+
+/*
+ * Descriptor for a read helper request.  This is used to make multiple I/O
+ * requests on a variety of sources and then stitch the result together.
+ */
+struct netfs_read_request {
+	struct work_struct	work;
+	struct inode		*inode;		/* The file being accessed */
+	struct address_space	*mapping;	/* The mapping being accessed */
+	struct netfs_cache_resources cache_resources;
+	struct list_head	subrequests;	/* Requests to fetch I/O from disk or net */
+	void			*netfs_priv;	/* Private data for the netfs */
+	unsigned int		debug_id;
+	unsigned int		cookie_debug_id;
+	atomic_t		nr_rd_ops;	/* Number of read ops in progress */
+	atomic_t		nr_wr_ops;	/* Number of write ops in progress */
+	size_t			submitted;	/* Amount submitted for I/O so far */
+	size_t			len;		/* Length of the request */
+	short			error;		/* 0 or error that occurred */
+	loff_t			i_size;		/* Size of the file */
+	loff_t			start;		/* Start position */
+	pgoff_t			no_unlock_page;	/* Don't unlock this page after read */
+	refcount_t		usage;
+	unsigned long		flags;
+#define NETFS_RREQ_INCOMPLETE_IO	0	/* Some ioreqs terminated short or with error */
+#define NETFS_RREQ_WRITE_TO_CACHE	1	/* Need to write to the cache */
+#define NETFS_RREQ_NO_UNLOCK_PAGE	2	/* Don't unlock no_unlock_page on completion */
+#define NETFS_RREQ_DONT_UNLOCK_PAGES	3	/* Don't unlock the pages on completion */
+#define NETFS_RREQ_FAILED		4	/* The request failed */
+#define NETFS_RREQ_IN_PROGRESS		5	/* Unlocked when the request completes */
+	const struct netfs_read_request_ops *netfs_ops;
+};
+
+/*
+ * Operations the network filesystem can/must provide to the helpers.
+ */
+struct netfs_read_request_ops {
+	bool (*is_cache_enabled)(struct inode *inode);
+	void (*init_rreq)(struct netfs_read_request *rreq, struct file *file);
+	int (*begin_cache_operation)(struct netfs_read_request *rreq);
+	void (*expand_readahead)(struct netfs_read_request *rreq);
+	bool (*clamp_length)(struct netfs_read_subrequest *subreq);
+	void (*issue_op)(struct netfs_read_subrequest *subreq);
+	bool (*is_still_valid)(struct netfs_read_request *rreq);
+	int (*check_write_begin)(struct file *file, loff_t pos, unsigned len,
+				 struct page *page, void **_fsdata);
+	void (*done)(struct netfs_read_request *rreq);
+	void (*cleanup)(struct address_space *mapping, void *netfs_priv);
+};
+
+/*
+ * Table of operations for access to a cache.  This is obtained by
+ * rreq->ops->begin_cache_operation().
+ */
+struct netfs_cache_ops {
+	/* End an operation */
+	void (*end_operation)(struct netfs_cache_resources *cres);
+
+	/* Read data from the cache */
+	int (*read)(struct netfs_cache_resources *cres,
+		    loff_t start_pos,
+		    struct iov_iter *iter,
+		    bool seek_data,
+		    netfs_io_terminated_t term_func,
+		    void *term_func_priv);
+
+	/* Write data to the cache */
+	int (*write)(struct netfs_cache_resources *cres,
+		     loff_t start_pos,
+		     struct iov_iter *iter,
+		     netfs_io_terminated_t term_func,
+		     void *term_func_priv);
+
+	/* Expand readahead request */
+	void (*expand_readahead)(struct netfs_cache_resources *cres,
+				 loff_t *_start, size_t *_len, loff_t i_size);
+
+	/* Prepare a read operation, shortening it to a cached/uncached
+	 * boundary as appropriate.
+	 */
+	enum netfs_read_source (*prepare_read)(struct netfs_read_subrequest *subreq,
+					       loff_t i_size);
+
+	/* Prepare a write operation, working out what part of the write we can
+	 * actually do.
+	 */
+	int (*prepare_write)(struct netfs_cache_resources *cres,
+			     loff_t *_start, size_t *_len, loff_t i_size);
+};
+
+struct readahead_control;
+extern void netfs_readahead(struct readahead_control *,
+			    const struct netfs_read_request_ops *,
+			    void *);
+extern int netfs_readpage(struct file *,
+			  struct page *,
+			  const struct netfs_read_request_ops *,
+			  void *);
+extern int netfs_write_begin(struct file *, struct address_space *,
+			     loff_t, unsigned int, unsigned int, struct page **,
+			     void **,
+			     const struct netfs_read_request_ops *,
+			     void *);
+
+extern void netfs_subreq_terminated(struct netfs_read_subrequest *, ssize_t, bool);
+extern void netfs_stats_show(struct seq_file *);
+
+#endif /* _LINUX_NETFS_H */

include/linux/pagemap.h

@@ -688,6 +688,26 @@ void wait_for_stable_page(struct page *page);
 
 void page_endio(struct page *page, bool is_write, int err);
 
+/**
+ * set_page_private_2 - Set PG_private_2 on a page and take a ref
+ * @page: The page.
+ *
+ * Set the PG_private_2 flag on a page and take the reference needed for the VM
+ * to handle its lifetime correctly.  This sets the flag and takes the
+ * reference unconditionally, so care must be taken not to set the flag again
+ * if it's already set.
+ */
+static inline void set_page_private_2(struct page *page)
+{
+	page = compound_head(page);
+	get_page(page);
+	SetPagePrivate2(page);
+}
+
+void end_page_private_2(struct page *page);
+void wait_on_page_private_2(struct page *page);
+int wait_on_page_private_2_killable(struct page *page);
+
 /*
  * Add an arbitrary waiter to a page's wait queue
  */
@@ -792,20 +812,23 @@ static inline int add_to_page_cache(struct page *page,
  * @file: The file, used primarily by network filesystems for authentication.
  *	  May be NULL if invoked internally by the filesystem.
  * @mapping: Readahead this filesystem object.
+ * @ra: File readahead state.  May be NULL.
  */
 struct readahead_control {
 	struct file *file;
 	struct address_space *mapping;
+	struct file_ra_state *ra;
 /* private: use the readahead_* accessors instead */
 	pgoff_t _index;
 	unsigned int _nr_pages;
 	unsigned int _batch_count;
 };
 
-#define DEFINE_READAHEAD(rac, f, m, i)					\
-	struct readahead_control rac = {				\
+#define DEFINE_READAHEAD(ractl, f, r, m, i)				\
+	struct readahead_control ractl = {				\
 		.file = f,						\
 		.mapping = m,						\
+		.ra = r,						\
 		._index = i,						\
 	}
 
@@ -813,10 +836,11 @@ struct readahead_control {
 
 void page_cache_ra_unbounded(struct readahead_control *,
 		unsigned long nr_to_read, unsigned long lookahead_count);
-void page_cache_sync_ra(struct readahead_control *, struct file_ra_state *,
+void page_cache_sync_ra(struct readahead_control *, unsigned long req_count);
+void page_cache_async_ra(struct readahead_control *, struct page *,
 		unsigned long req_count);
-void page_cache_async_ra(struct readahead_control *, struct file_ra_state *,
-		struct page *, unsigned long req_count);
+void readahead_expand(struct readahead_control *ractl,
+		      loff_t new_start, size_t new_len);
 
 /**
  * page_cache_sync_readahead - generic file readahead
@@ -836,8 +860,8 @@ void page_cache_sync_readahead(struct address_space *mapping,
 		struct file_ra_state *ra, struct file *file, pgoff_t index,
 		unsigned long req_count)
 {
-	DEFINE_READAHEAD(ractl, file, mapping, index);
-	page_cache_sync_ra(&ractl, ra, req_count);
+	DEFINE_READAHEAD(ractl, file, ra, mapping, index);
+	page_cache_sync_ra(&ractl, req_count);
 }
 
 /**
@@ -859,8 +883,8 @@ void page_cache_async_readahead(struct address_space *mapping,
 		struct file_ra_state *ra, struct file *file,
 		struct page *page, pgoff_t index, unsigned long req_count)
 {
-	DEFINE_READAHEAD(ractl, file, mapping, index);
-	page_cache_async_ra(&ractl, ra, page, req_count);
+	DEFINE_READAHEAD(ractl, file, ra, mapping, index);
+	page_cache_async_ra(&ractl, page, req_count);
 }
 
 /**

include/linux/uio.h

@@ -24,6 +24,7 @@ enum iter_type {
 	ITER_BVEC = 16,
 	ITER_PIPE = 32,
 	ITER_DISCARD = 64,
+	ITER_XARRAY = 128,
 };
 
 struct iov_iter {
@@ -39,6 +40,7 @@ struct iov_iter {
 		const struct iovec *iov;
 		const struct kvec *kvec;
 		const struct bio_vec *bvec;
+		struct xarray *xarray;
 		struct pipe_inode_info *pipe;
 	};
 	union {
@@ -47,6 +49,7 @@ struct iov_iter {
 			unsigned int head;
 			unsigned int start_head;
 		};
+		loff_t xarray_start;
 	};
 };
 
@@ -80,6 +83,11 @@ static inline bool iov_iter_is_discard(const struct iov_iter *i)
 	return iov_iter_type(i) == ITER_DISCARD;
 }
 
+static inline bool iov_iter_is_xarray(const struct iov_iter *i)
+{
+	return iov_iter_type(i) == ITER_XARRAY;
+}
+
 static inline unsigned char iov_iter_rw(const struct iov_iter *i)
 {
 	return i->type & (READ | WRITE);
@@ -221,6 +229,8 @@ void iov_iter_bvec(struct iov_iter *i, unsigned int direction, const struct bio_
 void iov_iter_pipe(struct iov_iter *i, unsigned int direction, struct pipe_inode_info *pipe,
 			size_t count);
 void iov_iter_discard(struct iov_iter *i, unsigned int direction, size_t count);
+void iov_iter_xarray(struct iov_iter *i, unsigned int direction, struct xarray *xarray,
+		     loff_t start, size_t count);
 ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
 			size_t maxsize, unsigned maxpages, size_t *start);
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages,

include/trace/events/netfs.h

@@ -0,0 +1,261 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Network filesystem support module tracepoints
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM netfs
+
+#if !defined(_TRACE_NETFS_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_NETFS_H
+
+#include <linux/tracepoint.h>
+
+/*
+ * Define enums for tracing information.
+ */
+#ifndef __NETFS_DECLARE_TRACE_ENUMS_ONCE_ONLY
+#define __NETFS_DECLARE_TRACE_ENUMS_ONCE_ONLY
+
+enum netfs_read_trace {
+	netfs_read_trace_expanded,
+	netfs_read_trace_readahead,
+	netfs_read_trace_readpage,
+	netfs_read_trace_write_begin,
+};
+
+enum netfs_rreq_trace {
+	netfs_rreq_trace_assess,
+	netfs_rreq_trace_done,
+	netfs_rreq_trace_free,
+	netfs_rreq_trace_resubmit,
+	netfs_rreq_trace_unlock,
+	netfs_rreq_trace_unmark,
+	netfs_rreq_trace_write,
+};
+
+enum netfs_sreq_trace {
+	netfs_sreq_trace_download_instead,
+	netfs_sreq_trace_free,
+	netfs_sreq_trace_prepare,
+	netfs_sreq_trace_resubmit_short,
+	netfs_sreq_trace_submit,
+	netfs_sreq_trace_terminated,
+	netfs_sreq_trace_write,
+	netfs_sreq_trace_write_skip,
+	netfs_sreq_trace_write_term,
+};
+
+enum netfs_failure {
+	netfs_fail_check_write_begin,
+	netfs_fail_copy_to_cache,
+	netfs_fail_read,
+	netfs_fail_short_readpage,
+	netfs_fail_short_write_begin,
+	netfs_fail_prepare_write,
+};
+
+#endif
+
+#define netfs_read_traces					\
+	EM(netfs_read_trace_expanded,		"EXPANDED ")	\
+	EM(netfs_read_trace_readahead,		"READAHEAD")	\
+	EM(netfs_read_trace_readpage,		"READPAGE ")	\
+	E_(netfs_read_trace_write_begin,	"WRITEBEGN")
+
+#define netfs_rreq_traces					\
+	EM(netfs_rreq_trace_assess,		"ASSESS")	\
+	EM(netfs_rreq_trace_done,		"DONE  ")	\
+	EM(netfs_rreq_trace_free,		"FREE  ")	\
+	EM(netfs_rreq_trace_resubmit,		"RESUBM")	\
+	EM(netfs_rreq_trace_unlock,		"UNLOCK")	\
+	EM(netfs_rreq_trace_unmark,		"UNMARK")	\
+	E_(netfs_rreq_trace_write,		"WRITE ")
+
+#define netfs_sreq_sources					\
+	EM(NETFS_FILL_WITH_ZEROES,		"ZERO")		\
+	EM(NETFS_DOWNLOAD_FROM_SERVER,		"DOWN")		\
+	EM(NETFS_READ_FROM_CACHE,		"READ")		\
+	E_(NETFS_INVALID_READ,			"INVL")		\
+
+#define netfs_sreq_traces					\
+	EM(netfs_sreq_trace_download_instead,	"RDOWN")	\
+	EM(netfs_sreq_trace_free,		"FREE ")	\
+	EM(netfs_sreq_trace_prepare,		"PREP ")	\
+	EM(netfs_sreq_trace_resubmit_short,	"SHORT")	\
+	EM(netfs_sreq_trace_submit,		"SUBMT")	\
+	EM(netfs_sreq_trace_terminated,		"TERM ")	\
+	EM(netfs_sreq_trace_write,		"WRITE")	\
+	EM(netfs_sreq_trace_write_skip,		"SKIP ")	\
+	E_(netfs_sreq_trace_write_term,		"WTERM")
+
+#define netfs_failures							\
+	EM(netfs_fail_check_write_begin,	"check-write-begin")	\
+	EM(netfs_fail_copy_to_cache,		"copy-to-cache")	\
+	EM(netfs_fail_read,			"read")			\
+	EM(netfs_fail_short_readpage,		"short-readpage")	\
+	EM(netfs_fail_short_write_begin,	"short-write-begin")	\
+	E_(netfs_fail_prepare_write,		"prep-write")
+
+
+/*
+ * Export enum symbols via userspace.
+ */
+#undef EM
+#undef E_
+#define EM(a, b) TRACE_DEFINE_ENUM(a);
+#define E_(a, b) TRACE_DEFINE_ENUM(a);
+
+netfs_read_traces;
+netfs_rreq_traces;
+netfs_sreq_sources;
+netfs_sreq_traces;
+netfs_failures;
+
+/*
+ * Now redefine the EM() and E_() macros to map the enums to the strings that
+ * will be printed in the output.
+ */
+#undef EM
+#undef E_
+#define EM(a, b)	{ a, b },
+#define E_(a, b)	{ a, b }
+
+TRACE_EVENT(netfs_read,
+	    TP_PROTO(struct netfs_read_request *rreq,
+		     loff_t start, size_t len,
+		     enum netfs_read_trace what),
+
+	    TP_ARGS(rreq, start, len, what),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		rreq		)
+		    __field(unsigned int,		cookie		)
+		    __field(loff_t,			start		)
+		    __field(size_t,			len		)
+		    __field(enum netfs_read_trace,	what		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->rreq	= rreq->debug_id;
+		    __entry->cookie	= rreq->cookie_debug_id;
+		    __entry->start	= start;
+		    __entry->len	= len;
+		    __entry->what	= what;
+			   ),
+
+	    TP_printk("R=%08x %s c=%08x s=%llx %zx",
+		      __entry->rreq,
+		      __print_symbolic(__entry->what, netfs_read_traces),
+		      __entry->cookie,
+		      __entry->start, __entry->len)
+	    );
+
+TRACE_EVENT(netfs_rreq,
+	    TP_PROTO(struct netfs_read_request *rreq,
+		     enum netfs_rreq_trace what),
+
+	    TP_ARGS(rreq, what),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		rreq		)
+		    __field(unsigned short,		flags		)
+		    __field(enum netfs_rreq_trace,	what		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->rreq	= rreq->debug_id;
+		    __entry->flags	= rreq->flags;
+		    __entry->what	= what;
+			   ),
+
+	    TP_printk("R=%08x %s f=%02x",
+		      __entry->rreq,
+		      __print_symbolic(__entry->what, netfs_rreq_traces),
+		      __entry->flags)
+	    );
+
+TRACE_EVENT(netfs_sreq,
+	    TP_PROTO(struct netfs_read_subrequest *sreq,
+		     enum netfs_sreq_trace what),
+
+	    TP_ARGS(sreq, what),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		rreq		)
+		    __field(unsigned short,		index		)
+		    __field(short,			error		)
+		    __field(unsigned short,		flags		)
+		    __field(enum netfs_read_source,	source		)
+		    __field(enum netfs_sreq_trace,	what		)
+		    __field(size_t,			len		)
+		    __field(size_t,			transferred	)
+		    __field(loff_t,			start		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->rreq	= sreq->rreq->debug_id;
+		    __entry->index	= sreq->debug_index;
+		    __entry->error	= sreq->error;
+		    __entry->flags	= sreq->flags;
+		    __entry->source	= sreq->source;
+		    __entry->what	= what;
+		    __entry->len	= sreq->len;
+		    __entry->transferred = sreq->transferred;
+		    __entry->start	= sreq->start;
+			   ),
+
+	    TP_printk("R=%08x[%u] %s %s f=%02x s=%llx %zx/%zx e=%d",
+		      __entry->rreq, __entry->index,
+		      __print_symbolic(__entry->what, netfs_sreq_traces),
+		      __print_symbolic(__entry->source, netfs_sreq_sources),
+		      __entry->flags,
+		      __entry->start, __entry->transferred, __entry->len,
+		      __entry->error)
+	    );
+
+TRACE_EVENT(netfs_failure,
+	    TP_PROTO(struct netfs_read_request *rreq,
+		     struct netfs_read_subrequest *sreq,
+		     int error, enum netfs_failure what),
+
+	    TP_ARGS(rreq, sreq, error, what),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		rreq		)
+		    __field(unsigned short,		index		)
+		    __field(short,			error		)
+		    __field(unsigned short,		flags		)
+		    __field(enum netfs_read_source,	source		)
+		    __field(enum netfs_failure,		what		)
+		    __field(size_t,			len		)
+		    __field(size_t,			transferred	)
+		    __field(loff_t,			start		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->rreq	= rreq->debug_id;
+		    __entry->index	= sreq ? sreq->debug_index : 0;
+		    __entry->error	= error;
+		    __entry->flags	= sreq ? sreq->flags : 0;
+		    __entry->source	= sreq ? sreq->source : NETFS_INVALID_READ;
+		    __entry->what	= what;
+		    __entry->len	= sreq ? sreq->len : 0;
+		    __entry->transferred = sreq ? sreq->transferred : 0;
+		    __entry->start	= sreq ? sreq->start : 0;
+			   ),
+
+	    TP_printk("R=%08x[%u] %s f=%02x s=%llx %zx/%zx %s e=%d",
+		      __entry->rreq, __entry->index,
+		      __print_symbolic(__entry->source, netfs_sreq_sources),
+		      __entry->flags,
+		      __entry->start, __entry->transferred, __entry->len,
+		      __print_symbolic(__entry->what, netfs_failures),
+		      __entry->error)
+	    );
+
+#endif /* _TRACE_NETFS_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

lib/iov_iter.c

@@ -76,7 +76,44 @@
 	}						\
 }
 
-#define iterate_all_kinds(i, n, v, I, B, K) {			\
+#define iterate_xarray(i, n, __v, skip, STEP) {		\
+	struct page *head = NULL;				\
+	size_t wanted = n, seg, offset;				\
+	loff_t start = i->xarray_start + skip;			\
+	pgoff_t index = start >> PAGE_SHIFT;			\
+	int j;							\
+								\
+	XA_STATE(xas, i->xarray, index);			\
+								\
+	rcu_read_lock();						\
+	xas_for_each(&xas, head, ULONG_MAX) {				\
+		if (xas_retry(&xas, head))				\
+			continue;					\
+		if (WARN_ON(xa_is_value(head)))				\
+			break;						\
+		if (WARN_ON(PageHuge(head)))				\
+			break;						\
+		for (j = (head->index < index) ? index - head->index : 0; \
+		     j < thp_nr_pages(head); j++) {			\
+			__v.bv_page = head + j;				\
+			offset = (i->xarray_start + skip) & ~PAGE_MASK;	\
+			seg = PAGE_SIZE - offset;			\
+			__v.bv_offset = offset;				\
+			__v.bv_len = min(n, seg);			\
+			(void)(STEP);					\
+			n -= __v.bv_len;				\
+			skip += __v.bv_len;				\
+			if (n == 0)					\
+				break;					\
+		}							\
+		if (n == 0)						\
+			break;						\
+	}							\
+	rcu_read_unlock();					\
+	n = wanted - n;						\
+}
+
+#define iterate_all_kinds(i, n, v, I, B, K, X) {		\
 	if (likely(n)) {					\
 		size_t skip = i->iov_offset;			\
 		if (unlikely(i->type & ITER_BVEC)) {		\
@@ -88,6 +125,9 @@
 			struct kvec v;				\
 			iterate_kvec(i, n, v, kvec, skip, (K))	\
 		} else if (unlikely(i->type & ITER_DISCARD)) {	\
+		} else if (unlikely(i->type & ITER_XARRAY)) {	\
+			struct bio_vec v;			\
+			iterate_xarray(i, n, v, skip, (X));	\
 		} else {					\
 			const struct iovec *iov;		\
 			struct iovec v;				\
@@ -96,7 +136,7 @@
 	}							\
 }
 
-#define iterate_and_advance(i, n, v, I, B, K) {			\
+#define iterate_and_advance(i, n, v, I, B, K, X) {		\
 	if (unlikely(i->count < n))				\
 		n = i->count;					\
 	if (i->count) {						\
@@ -121,6 +161,9 @@
 			i->kvec = kvec;				\
 		} else if (unlikely(i->type & ITER_DISCARD)) {	\
 			skip += n;				\
+		} else if (unlikely(i->type & ITER_XARRAY)) {	\
+			struct bio_vec v;			\
+			iterate_xarray(i, n, v, skip, (X))	\
 		} else {					\
 			const struct iovec *iov;		\
 			struct iovec v;				\
@@ -622,7 +665,9 @@ size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 		copyout(v.iov_base, (from += v.iov_len) - v.iov_len, v.iov_len),
 		memcpy_to_page(v.bv_page, v.bv_offset,
 			       (from += v.bv_len) - v.bv_len, v.bv_len),
-		memcpy(v.iov_base, (from += v.iov_len) - v.iov_len, v.iov_len)
+		memcpy(v.iov_base, (from += v.iov_len) - v.iov_len, v.iov_len),
+		memcpy_to_page(v.bv_page, v.bv_offset,
+			       (from += v.bv_len) - v.bv_len, v.bv_len)
 	)
 
 	return bytes;
@@ -738,6 +783,18 @@ size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
 			bytes = curr_addr - s_addr - rem;
 			return bytes;
 		}
+		}),
+		({
+		rem = copy_mc_to_page(v.bv_page, v.bv_offset,
+				      (from += v.bv_len) - v.bv_len, v.bv_len);
+		if (rem) {
+			curr_addr = (unsigned long) from;
+			bytes = curr_addr - s_addr - rem;
+			rcu_read_unlock();
+			i->iov_offset += bytes;
+			i->count -= bytes;
+			return bytes;
+		}
 		})
 	)
 
@@ -759,7 +816,9 @@ size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
 		copyin((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
 		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
-		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
+		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
+		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 
 	return bytes;
@@ -785,7 +844,9 @@ bool _copy_from_iter_full(void *addr, size_t bytes, struct iov_iter *i)
 		0;}),
 		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
-		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
+		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
+		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 
 	iov_iter_advance(i, bytes);
@@ -805,7 +866,9 @@ size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 					 v.iov_base, v.iov_len),
 		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
-		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
+		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
+		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 
 	return bytes;
@@ -840,7 +903,9 @@ size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
 		memcpy_page_flushcache((to += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
 		memcpy_flushcache((to += v.iov_len) - v.iov_len, v.iov_base,
-			v.iov_len)
+			v.iov_len),
+		memcpy_page_flushcache((to += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 
 	return bytes;
@@ -864,7 +929,9 @@ bool _copy_from_iter_full_nocache(void *addr, size_t bytes, struct iov_iter *i)
 		0;}),
 		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
-		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
+		memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
+		memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 
 	iov_iter_advance(i, bytes);
@@ -901,7 +968,7 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 {
 	if (unlikely(!page_copy_sane(page, offset, bytes)))
 		return 0;
-	if (i->type & (ITER_BVEC|ITER_KVEC)) {
+	if (i->type & (ITER_BVEC | ITER_KVEC | ITER_XARRAY)) {
 		void *kaddr = kmap_atomic(page);
 		size_t wanted = copy_to_iter(kaddr + offset, bytes, i);
 		kunmap_atomic(kaddr);
@@ -924,7 +991,7 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 		WARN_ON(1);
 		return 0;
 	}
-	if (i->type & (ITER_BVEC|ITER_KVEC)) {
+	if (i->type & (ITER_BVEC | ITER_KVEC | ITER_XARRAY)) {
 		void *kaddr = kmap_atomic(page);
 		size_t wanted = _copy_from_iter(kaddr + offset, bytes, i);
 		kunmap_atomic(kaddr);
@@ -968,7 +1035,8 @@ size_t iov_iter_zero(size_t bytes, struct iov_iter *i)
 	iterate_and_advance(i, bytes, v,
 		clear_user(v.iov_base, v.iov_len),
 		memzero_page(v.bv_page, v.bv_offset, v.bv_len),
-		memset(v.iov_base, 0, v.iov_len)
+		memset(v.iov_base, 0, v.iov_len),
+		memzero_page(v.bv_page, v.bv_offset, v.bv_len)
 	)
 
 	return bytes;
@@ -992,7 +1060,9 @@ size_t iov_iter_copy_from_user_atomic(struct page *page,
 		copyin((p += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
 		memcpy_from_page((p += v.bv_len) - v.bv_len, v.bv_page,
 				 v.bv_offset, v.bv_len),
-		memcpy((p += v.iov_len) - v.iov_len, v.iov_base, v.iov_len)
+		memcpy((p += v.iov_len) - v.iov_len, v.iov_base, v.iov_len),
+		memcpy_from_page((p += v.bv_len) - v.bv_len, v.bv_page,
+				 v.bv_offset, v.bv_len)
 	)
 	kunmap_atomic(kaddr);
 	return bytes;
@@ -1078,11 +1148,17 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
 		i->count -= size;
 		return;
 	}
+	if (unlikely(iov_iter_is_xarray(i))) {
+		size = min(size, i->count);
+		i->iov_offset += size;
+		i->count -= size;
+		return;
+	}
 	if (iov_iter_is_bvec(i)) {
 		iov_iter_bvec_advance(i, size);
 		return;
 	}
-	iterate_and_advance(i, size, v, 0, 0, 0)
+	iterate_and_advance(i, size, v, 0, 0, 0, 0)
 }
 EXPORT_SYMBOL(iov_iter_advance);
 
@@ -1126,7 +1202,12 @@ void iov_iter_revert(struct iov_iter *i, size_t unroll)
 		return;
 	}
 	unroll -= i->iov_offset;
-	if (iov_iter_is_bvec(i)) {
+	if (iov_iter_is_xarray(i)) {
+		BUG(); /* We should never go beyond the start of the specified
+			* range since we might then be straying into pages that
+			* aren't pinned.
+			*/
+	} else if (iov_iter_is_bvec(i)) {
 		const struct bio_vec *bvec = i->bvec;
 		while (1) {
 			size_t n = (--bvec)->bv_len;
@@ -1163,9 +1244,9 @@ size_t iov_iter_single_seg_count(const struct iov_iter *i)
 		return i->count;	// it is a silly place, anyway
 	if (i->nr_segs == 1)
 		return i->count;
-	if (unlikely(iov_iter_is_discard(i)))
+	if (unlikely(iov_iter_is_discard(i) || iov_iter_is_xarray(i)))
 		return i->count;
-	else if (iov_iter_is_bvec(i))
+	if (iov_iter_is_bvec(i))
 		return min(i->count, i->bvec->bv_len - i->iov_offset);
 	else
 		return min(i->count, i->iov->iov_len - i->iov_offset);
@@ -1213,6 +1294,31 @@ void iov_iter_pipe(struct iov_iter *i, unsigned int direction,
 }
 EXPORT_SYMBOL(iov_iter_pipe);
 
+/**
+ * iov_iter_xarray - Initialise an I/O iterator to use the pages in an xarray
+ * @i: The iterator to initialise.
+ * @direction: The direction of the transfer.
+ * @xarray: The xarray to access.
+ * @start: The start file position.
+ * @count: The size of the I/O buffer in bytes.
+ *
+ * Set up an I/O iterator to either draw data out of the pages attached to an
+ * inode or to inject data into those pages.  The pages *must* be prevented
+ * from evaporation, either by taking a ref on them or locking them by the
+ * caller.
+ */
+void iov_iter_xarray(struct iov_iter *i, unsigned int direction,
+		     struct xarray *xarray, loff_t start, size_t count)
+{
+	BUG_ON(direction & ~1);
+	i->type = ITER_XARRAY | (direction & (READ | WRITE));
+	i->xarray = xarray;
+	i->xarray_start = start;
+	i->count = count;
+	i->iov_offset = 0;
+}
+EXPORT_SYMBOL(iov_iter_xarray);
+
 /**
  * iov_iter_discard - Initialise an I/O iterator that discards data
  * @i: The iterator to initialise.
@@ -1243,10 +1349,13 @@ unsigned long iov_iter_alignment(const struct iov_iter *i)
 			return size | i->iov_offset;
 		return size;
 	}
+	if (unlikely(iov_iter_is_xarray(i)))
+		return (i->xarray_start + i->iov_offset) | i->count;
 	iterate_all_kinds(i, size, v,
 		(res |= (unsigned long)v.iov_base | v.iov_len, 0),
 		res |= v.bv_offset | v.bv_len,
-		res |= (unsigned long)v.iov_base | v.iov_len
+		res |= (unsigned long)v.iov_base | v.iov_len,
+		res |= v.bv_offset | v.bv_len
 	)
 	return res;
 }
@@ -1268,7 +1377,9 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
 		(res |= (!res ? 0 : (unsigned long)v.bv_offset) |
 			(size != v.bv_len ? size : 0)),
 		(res |= (!res ? 0 : (unsigned long)v.iov_base) |
-			(size != v.iov_len ? size : 0))
+			(size != v.iov_len ? size : 0)),
+		(res |= (!res ? 0 : (unsigned long)v.bv_offset) |
+			(size != v.bv_len ? size : 0))
 		);
 	return res;
 }
@@ -1318,6 +1429,75 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
 	return __pipe_get_pages(i, min(maxsize, capacity), pages, iter_head, start);
 }
 
+static ssize_t iter_xarray_populate_pages(struct page **pages, struct xarray *xa,
+					  pgoff_t index, unsigned int nr_pages)
+{
+	XA_STATE(xas, xa, index);
+	struct page *page;
+	unsigned int ret = 0;
+
+	rcu_read_lock();
+	for (page = xas_load(&xas); page; page = xas_next(&xas)) {
+		if (xas_retry(&xas, page))
+			continue;
+
+		/* Has the page moved or been split? */
+		if (unlikely(page != xas_reload(&xas))) {
+			xas_reset(&xas);
+			continue;
+		}
+
+		pages[ret] = find_subpage(page, xas.xa_index);
+		get_page(pages[ret]);
+		if (++ret == nr_pages)
+			break;
+	}
+	rcu_read_unlock();
+	return ret;
+}
+
+static ssize_t iter_xarray_get_pages(struct iov_iter *i,
+				     struct page **pages, size_t maxsize,
+				     unsigned maxpages, size_t *_start_offset)
+{
+	unsigned nr, offset;
+	pgoff_t index, count;
+	size_t size = maxsize, actual;
+	loff_t pos;
+
+	if (!size || !maxpages)
+		return 0;
+
+	pos = i->xarray_start + i->iov_offset;
+	index = pos >> PAGE_SHIFT;
+	offset = pos & ~PAGE_MASK;
+	*_start_offset = offset;
+
+	count = 1;
+	if (size > PAGE_SIZE - offset) {
+		size -= PAGE_SIZE - offset;
+		count += size >> PAGE_SHIFT;
+		size &= ~PAGE_MASK;
+		if (size)
+			count++;
+	}
+
+	if (count > maxpages)
+		count = maxpages;
+
+	nr = iter_xarray_populate_pages(pages, i->xarray, index, count);
+	if (nr == 0)
+		return 0;
+
+	actual = PAGE_SIZE * nr;
+	actual -= offset;
+	if (nr == count && size > 0) {
+		unsigned last_offset = (nr > 1) ? 0 : offset;
+		actual -= PAGE_SIZE - (last_offset + size);
+	}
+	return actual;
+}
+
 ssize_t iov_iter_get_pages(struct iov_iter *i,
 		   struct page **pages, size_t maxsize, unsigned maxpages,
 		   size_t *start)
@@ -1327,6 +1507,8 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 
 	if (unlikely(iov_iter_is_pipe(i)))
 		return pipe_get_pages(i, pages, maxsize, maxpages, start);
+	if (unlikely(iov_iter_is_xarray(i)))
+		return iter_xarray_get_pages(i, pages, maxsize, maxpages, start);
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
@@ -1353,7 +1535,8 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		return v.bv_len;
 	}),({
 		return -EFAULT;
-	})
+	}),
+	0
 	)
 	return 0;
 }
@@ -1397,6 +1580,51 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 	return n;
 }
 
+static ssize_t iter_xarray_get_pages_alloc(struct iov_iter *i,
+					   struct page ***pages, size_t maxsize,
+					   size_t *_start_offset)
+{
+	struct page **p;
+	unsigned nr, offset;
+	pgoff_t index, count;
+	size_t size = maxsize, actual;
+	loff_t pos;
+
+	if (!size)
+		return 0;
+
+	pos = i->xarray_start + i->iov_offset;
+	index = pos >> PAGE_SHIFT;
+	offset = pos & ~PAGE_MASK;
+	*_start_offset = offset;
+
+	count = 1;
+	if (size > PAGE_SIZE - offset) {
+		size -= PAGE_SIZE - offset;
+		count += size >> PAGE_SHIFT;
+		size &= ~PAGE_MASK;
+		if (size)
+			count++;
+	}
+
+	p = get_pages_array(count);
+	if (!p)
+		return -ENOMEM;
+	*pages = p;
+
+	nr = iter_xarray_populate_pages(p, i->xarray, index, count);
+	if (nr == 0)
+		return 0;
+
+	actual = PAGE_SIZE * nr;
+	actual -= offset;
+	if (nr == count && size > 0) {
+		unsigned last_offset = (nr > 1) ? 0 : offset;
+		actual -= PAGE_SIZE - (last_offset + size);
+	}
+	return actual;
+}
+
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		   struct page ***pages, size_t maxsize,
 		   size_t *start)
@@ -1408,6 +1636,8 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 
 	if (unlikely(iov_iter_is_pipe(i)))
 		return pipe_get_pages_alloc(i, pages, maxsize, start);
+	if (unlikely(iov_iter_is_xarray(i)))
+		return iter_xarray_get_pages_alloc(i, pages, maxsize, start);
 	if (unlikely(iov_iter_is_discard(i)))
 		return -EFAULT;
 
@@ -1440,7 +1670,7 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 		return v.bv_len;
 	}),({
 		return -EFAULT;
-	})
+	}), 0
 	)
 	return 0;
 }
@@ -1478,6 +1708,13 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
 				      v.iov_base, v.iov_len,
 				      sum, off);
 		off += v.iov_len;
+	}), ({
+		char *p = kmap_atomic(v.bv_page);
+		sum = csum_and_memcpy((to += v.bv_len) - v.bv_len,
+				      p + v.bv_offset, v.bv_len,
+				      sum, off);
+		kunmap_atomic(p);
+		off += v.bv_len;
 	})
 	)
 	*csum = sum;
@@ -1519,6 +1756,13 @@ bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum,
 				      v.iov_base, v.iov_len,
 				      sum, off);
 		off += v.iov_len;
+	}), ({
+		char *p = kmap_atomic(v.bv_page);
+		sum = csum_and_memcpy((to += v.bv_len) - v.bv_len,
+				      p + v.bv_offset, v.bv_len,
+				      sum, off);
+		kunmap_atomic(p);
+		off += v.bv_len;
 	})
 	)
 	*csum = sum;
@@ -1565,6 +1809,13 @@ size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *_csstate,
 				     (from += v.iov_len) - v.iov_len,
 				     v.iov_len, sum, off);
 		off += v.iov_len;
+	}), ({
+		char *p = kmap_atomic(v.bv_page);
+		sum = csum_and_memcpy(p + v.bv_offset,
+				      (from += v.bv_len) - v.bv_len,
+				      v.bv_len, sum, off);
+		kunmap_atomic(p);
+		off += v.bv_len;
 	})
 	)
 	csstate->csum = sum;
@@ -1615,6 +1866,21 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
 		npages = pipe_space_for_user(iter_head, pipe->tail, pipe);
 		if (npages >= maxpages)
 			return maxpages;
+	} else if (unlikely(iov_iter_is_xarray(i))) {
+		unsigned offset;
+
+		offset = (i->xarray_start + i->iov_offset) & ~PAGE_MASK;
+
+		npages = 1;
+		if (size > PAGE_SIZE - offset) {
+			size -= PAGE_SIZE - offset;
+			npages += size >> PAGE_SHIFT;
+			size &= ~PAGE_MASK;
+			if (size)
+				npages++;
+		}
+		if (npages >= maxpages)
+			return maxpages;
 	} else iterate_all_kinds(i, size, v, ({
 		unsigned long p = (unsigned long)v.iov_base;
 		npages += DIV_ROUND_UP(p + v.iov_len, PAGE_SIZE)
@@ -1631,7 +1897,8 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
 			- p / PAGE_SIZE;
 		if (npages >= maxpages)
 			return maxpages;
-	})
+	}),
+	0
 	)
 	return npages;
 }
@@ -1644,7 +1911,7 @@ const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
 		WARN_ON(1);
 		return NULL;
 	}
-	if (unlikely(iov_iter_is_discard(new)))
+	if (unlikely(iov_iter_is_discard(new) || iov_iter_is_xarray(new)))
 		return NULL;
 	if (iov_iter_is_bvec(new))
 		return new->bvec = kmemdup(new->bvec,
@@ -1849,7 +2116,12 @@ int iov_iter_for_each_range(struct iov_iter *i, size_t bytes,
 		kunmap(v.bv_page);
 		err;}), ({
 		w = v;
-		err = f(&w, context);})
+		err = f(&w, context);}), ({
+		w.iov_base = kmap(v.bv_page) + v.bv_offset;
+		w.iov_len = v.bv_len;
+		err = f(&w, context);
+		kunmap(v.bv_page);
+		err;})
 	)
 	return err;
 }

mm/filemap.c

@@ -1432,6 +1432,67 @@ void unlock_page(struct page *page)
 }
 EXPORT_SYMBOL(unlock_page);
 
+/**
+ * end_page_private_2 - Clear PG_private_2 and release any waiters
+ * @page: The page
+ *
+ * Clear the PG_private_2 bit on a page and wake up any sleepers waiting for
+ * this.  The page ref held for PG_private_2 being set is released.
+ *
+ * This is, for example, used when a netfs page is being written to a local
+ * disk cache, thereby allowing writes to the cache for the same page to be
+ * serialised.
+ */
+void end_page_private_2(struct page *page)
+{
+	page = compound_head(page);
+	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
+	clear_bit_unlock(PG_private_2, &page->flags);
+	wake_up_page_bit(page, PG_private_2);
+	put_page(page);
+}
+EXPORT_SYMBOL(end_page_private_2);
+
+/**
+ * wait_on_page_private_2 - Wait for PG_private_2 to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_private_2 (aka PG_fscache) to be cleared on a page.
+ */
+void wait_on_page_private_2(struct page *page)
+{
+	page = compound_head(page);
+	while (PagePrivate2(page))
+		wait_on_page_bit(page, PG_private_2);
+}
+EXPORT_SYMBOL(wait_on_page_private_2);
+
+/**
+ * wait_on_page_private_2_killable - Wait for PG_private_2 to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_private_2 (aka PG_fscache) to be cleared on a page or until a
+ * fatal signal is received by the calling task.
+ *
+ * Return:
+ * - 0 if successful.
+ * - -EINTR if a fatal signal was encountered.
+ */
+int wait_on_page_private_2_killable(struct page *page)
+{
+	int ret = 0;
+
+	page = compound_head(page);
+	while (PagePrivate2(page)) {
+		ret = wait_on_page_bit_killable(page, PG_private_2);
+		if (ret < 0)
+			break;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(wait_on_page_private_2_killable);
+
 /**
  * end_page_writeback - end writeback against a page
  * @page: the page
@@ -2778,7 +2839,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	struct file *file = vmf->vma->vm_file;
 	struct file_ra_state *ra = &file->f_ra;
 	struct address_space *mapping = file->f_mapping;
-	DEFINE_READAHEAD(ractl, file, mapping, vmf->pgoff);
+	DEFINE_READAHEAD(ractl, file, ra, mapping, vmf->pgoff);
 	struct file *fpin = NULL;
 	unsigned int mmap_miss;
 
@@ -2790,7 +2851,7 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 
 	if (vmf->vma->vm_flags & VM_SEQ_READ) {
 		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
-		page_cache_sync_ra(&ractl, ra, ra->ra_pages);
+		page_cache_sync_ra(&ractl, ra->ra_pages);
 		return fpin;
 	}
 

mm/internal.h

@@ -51,13 +51,12 @@ void unmap_page_range(struct mmu_gather *tlb,
 
 void do_page_cache_ra(struct readahead_control *, unsigned long nr_to_read,
 		unsigned long lookahead_size);
-void force_page_cache_ra(struct readahead_control *, struct file_ra_state *,
-		unsigned long nr);
+void force_page_cache_ra(struct readahead_control *, unsigned long nr);
 static inline void force_page_cache_readahead(struct address_space *mapping,
 		struct file *file, pgoff_t index, unsigned long nr_to_read)
 {
-	DEFINE_READAHEAD(ractl, file, mapping, index);
-	force_page_cache_ra(&ractl, &file->f_ra, nr_to_read);
+	DEFINE_READAHEAD(ractl, file, &file->f_ra, mapping, index);
+	force_page_cache_ra(&ractl, nr_to_read);
 }
 
 unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,

mm/readahead.c

@@ -198,8 +198,6 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 	for (i = 0; i < nr_to_read; i++) {
 		struct page *page = xa_load(&mapping->i_pages, index + i);
 
-		BUG_ON(index + i != ractl->_index + ractl->_nr_pages);
-
 		if (page && !xa_is_value(page)) {
 			/*
 			 * Page already present?  Kick off the current batch
@@ -210,6 +208,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 			 * not worth getting one just for that.
 			 */
 			read_pages(ractl, &page_pool, true);
+			i = ractl->_index + ractl->_nr_pages - index - 1;
 			continue;
 		}
 
@@ -223,6 +222,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 					gfp_mask) < 0) {
 			put_page(page);
 			read_pages(ractl, &page_pool, true);
+			i = ractl->_index + ractl->_nr_pages - index - 1;
 			continue;
 		}
 		if (i == nr_to_read - lookahead_size)
@@ -272,9 +272,10 @@ void do_page_cache_ra(struct readahead_control *ractl,
  * memory at once.
  */
 void force_page_cache_ra(struct readahead_control *ractl,
-		struct file_ra_state *ra, unsigned long nr_to_read)
+		unsigned long nr_to_read)
 {
 	struct address_space *mapping = ractl->mapping;
+	struct file_ra_state *ra = ractl->ra;
 	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
 	unsigned long max_pages, index;
 
@@ -433,10 +434,10 @@ static int try_context_readahead(struct address_space *mapping,
  * A minimal readahead algorithm for trivial sequential/random reads.
  */
 static void ondemand_readahead(struct readahead_control *ractl,
-		struct file_ra_state *ra, bool hit_readahead_marker,
-		unsigned long req_size)
+		bool hit_readahead_marker, unsigned long req_size)
 {
 	struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
+	struct file_ra_state *ra = ractl->ra;
 	unsigned long max_pages = ra->ra_pages;
 	unsigned long add_pages;
 	unsigned long index = readahead_index(ractl);
@@ -550,7 +551,7 @@ readit:
 }
 
 void page_cache_sync_ra(struct readahead_control *ractl,
-		struct file_ra_state *ra, unsigned long req_count)
+		unsigned long req_count)
 {
 	bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
 
@@ -560,7 +561,7 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 	 * read-ahead will do the right thing and limit the read to just the
 	 * requested range, which we'll set to 1 page for this case.
 	 */
-	if (!ra->ra_pages || blk_cgroup_congested()) {
+	if (!ractl->ra->ra_pages || blk_cgroup_congested()) {
 		if (!ractl->file)
 			return;
 		req_count = 1;
@@ -569,21 +570,20 @@ void page_cache_sync_ra(struct readahead_control *ractl,
 
 	/* be dumb */
 	if (do_forced_ra) {
-		force_page_cache_ra(ractl, ra, req_count);
+		force_page_cache_ra(ractl, req_count);
 		return;
 	}
 
 	/* do read-ahead */
-	ondemand_readahead(ractl, ra, false, req_count);
+	ondemand_readahead(ractl, false, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_ra);
 
 void page_cache_async_ra(struct readahead_control *ractl,
-		struct file_ra_state *ra, struct page *page,
-		unsigned long req_count)
+		struct page *page, unsigned long req_count)
 {
 	/* no read-ahead */
-	if (!ra->ra_pages)
+	if (!ractl->ra->ra_pages)
 		return;
 
 	/*
@@ -604,7 +604,7 @@ void page_cache_async_ra(struct readahead_control *ractl,
 		return;
 
 	/* do read-ahead */
-	ondemand_readahead(ractl, ra, true, req_count);
+	ondemand_readahead(ractl, true, req_count);
 }
 EXPORT_SYMBOL_GPL(page_cache_async_ra);
 
@@ -638,3 +638,78 @@ SYSCALL_DEFINE3(readahead, int, fd, loff_t, offset, size_t, count)
 {
 	return ksys_readahead(fd, offset, count);
 }
+
+/**
+ * readahead_expand - Expand a readahead request
+ * @ractl: The request to be expanded
+ * @new_start: The revised start
+ * @new_len: The revised size of the request
+ *
+ * Attempt to expand a readahead request outwards from the current size to the
+ * specified size by inserting locked pages before and after the current window
+ * to increase the size to the new window.  This may involve the insertion of
+ * THPs, in which case the window may get expanded even beyond what was
+ * requested.
+ *
+ * The algorithm will stop if it encounters a conflicting page already in the
+ * pagecache and leave a smaller expansion than requested.
+ *
+ * The caller must check for this by examining the revised @ractl object for a
+ * different expansion than was requested.
+ */
+void readahead_expand(struct readahead_control *ractl,
+		      loff_t new_start, size_t new_len)
+{
+	struct address_space *mapping = ractl->mapping;
+	struct file_ra_state *ra = ractl->ra;
+	pgoff_t new_index, new_nr_pages;
+	gfp_t gfp_mask = readahead_gfp_mask(mapping);
+
+	new_index = new_start / PAGE_SIZE;
+
+	/* Expand the leading edge downwards */
+	while (ractl->_index > new_index) {
+		unsigned long index = ractl->_index - 1;
+		struct page *page = xa_load(&mapping->i_pages, index);
+
+		if (page && !xa_is_value(page))
+			return; /* Page apparently present */
+
+		page = __page_cache_alloc(gfp_mask);
+		if (!page)
+			return;
+		if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
+			put_page(page);
+			return;
+		}
+
+		ractl->_nr_pages++;
+		ractl->_index = page->index;
+	}
+
+	new_len += new_start - readahead_pos(ractl);
+	new_nr_pages = DIV_ROUND_UP(new_len, PAGE_SIZE);
+
+	/* Expand the trailing edge upwards */
+	while (ractl->_nr_pages < new_nr_pages) {
+		unsigned long index = ractl->_index + ractl->_nr_pages;
+		struct page *page = xa_load(&mapping->i_pages, index);
+
+		if (page && !xa_is_value(page))
+			return; /* Page apparently present */
+
+		page = __page_cache_alloc(gfp_mask);
+		if (!page)
+			return;
+		if (add_to_page_cache_lru(page, mapping, index, gfp_mask) < 0) {
+			put_page(page);
+			return;
+		}
+		ractl->_nr_pages++;
+		if (ra) {
+			ra->size++;
+			ra->async_size++;
+		}
+	}
+}
+EXPORT_SYMBOL(readahead_expand);