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Author SHA1 Message Date
David Howells
1362729b16 FS-Cache: Simplify cookie retention for fscache_objects, fixing oops 
Simplify the way fscache cache objects retain their cookie.  The way I
implemented the cookie storage handling made synchronisation a pain (ie. the
object state machine can't rely on the cookie actually still being there).

Instead of the the object being detached from the cookie and the cookie being
freed in __fscache_relinquish_cookie(), we defer both operations:

 (*) The detachment of the object from the list in the cookie now takes place
     in fscache_drop_object() and is thus governed by the object state machine
     (fscache_detach_from_cookie() has been removed).

 (*) The release of the cookie is now in fscache_object_destroy() - which is
     called by the cache backend just before it frees the object.

This means that the fscache_cookie struct is now available to the cache all the
way through from ->alloc_object() to ->drop_object() and ->put_object() -
meaning that it's no longer necessary to take object->lock to guarantee access.

However, __fscache_relinquish_cookie() doesn't wait for the object to go all
the way through to destruction before letting the netfs proceed.  That would
massively slow down the netfs.  Since __fscache_relinquish_cookie() leaves the
cookie around, in must therefore break all attachments to the netfs - which
includes ->def, ->netfs_data and any outstanding page read/writes.

To handle this, struct fscache_cookie now has an n_active counter:

 (1) This starts off initialised to 1.

 (2) Any time the cache needs to get at the netfs data, it calls
     fscache_use_cookie() to increment it - if it is not zero.  If it was zero,
     then access is not permitted.

 (3) When the cache has finished with the data, it calls fscache_unuse_cookie()
     to decrement it.  This does a wake-up on it if it reaches 0.

 (4) __fscache_relinquish_cookie() decrements n_active and then waits for it to
     reach 0.  The initialisation to 1 in step (1) ensures that we only get
     wake ups when we're trying to get rid of the cookie.

This leaves __fscache_relinquish_cookie() a lot simpler.


***
This fixes a problem in the current code whereby if fscache_invalidate() is
followed sufficiently quickly by fscache_relinquish_cookie() then it is
possible for __fscache_relinquish_cookie() to have detached the cookie from the
object and cleared the pointer before a thread is dispatched to process the
invalidation state in the object state machine.

Since the pending write clearance was deferred to the invalidation state to
make it asynchronous, we need to either wait in relinquishment for the stores
tree to be cleared in the invalidation state or we need to handle the clearance
in relinquishment.

Further, if the relinquishment code does clear the tree, then the invalidation
state need to make the clearance contingent on still having the cookie to hand
(since that's where the tree is rooted) and we have to prevent the cookie from
disappearing for the duration.

This can lead to an oops like the following:

BUG: unable to handle kernel NULL pointer dereference at 000000000000000c
...
RIP: 0010:[<ffffffff8151023e>] _spin_lock+0xe/0x30
...
CR2: 000000000000000c ...
...
Process kslowd002 (...)
....
Call Trace:
 [<ffffffffa01c3278>] fscache_invalidate_writes+0x38/0xd0 [fscache]
 [<ffffffff810096f0>] ? __switch_to+0xd0/0x320
 [<ffffffff8105e759>] ? find_busiest_queue+0x69/0x150
 [<ffffffff8110ddd4>] ? slow_work_enqueue+0x104/0x180
 [<ffffffffa01c1303>] fscache_object_slow_work_execute+0x5e3/0x9d0 [fscache]
 [<ffffffff81096b67>] ? bit_waitqueue+0x17/0xd0
 [<ffffffff8110e233>] slow_work_execute+0x233/0x310
 [<ffffffff8110e515>] slow_work_thread+0x205/0x360
 [<ffffffff81096ca0>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff8110e310>] ? slow_work_thread+0x0/0x360
 [<ffffffff81096936>] kthread+0x96/0xa0
 [<ffffffff8100c0ca>] child_rip+0xa/0x20
 [<ffffffff810968a0>] ? kthread+0x0/0xa0
 [<ffffffff8100c0c0>] ? child_rip+0x0/0x20

The parameter to fscache_invalidate_writes() was object->cookie which is NULL.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-By: Milosz Tanski <milosz@adfin.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
 2013-06-19 14:16:47 +01:00
David Howells
5f4f9f4af1 CacheFiles: Downgrade the requirements passed to the allocator 
Downgrade the requirements passed to the allocator in the gfp flags parameter.
FS-Cache/CacheFiles can handle OOM conditions simply by aborting the attempt to
store an object or a page in the cache.

Signed-off-by: David Howells <dhowells@redhat.com>
 2012-12-20 21:58:25 +00:00
Tejun Heo
5a0e3ad6af include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h 
percpu.h is included by sched.h and module.h and thus ends up being
included when building most .c files.  percpu.h includes slab.h which
in turn includes gfp.h making everything defined by the two files
universally available and complicating inclusion dependencies.

percpu.h -> slab.h dependency is about to be removed.  Prepare for
this change by updating users of gfp and slab facilities include those
headers directly instead of assuming availability.  As this conversion
needs to touch large number of source files, the following script is
used as the basis of conversion.

  http://userweb.kernel.org/~tj/misc/slabh-sweep.py

The script does the followings.

* Scan files for gfp and slab usages and update includes such that
  only the necessary includes are there.  ie. if only gfp is used,
  gfp.h, if slab is used, slab.h.

* When the script inserts a new include, it looks at the include
  blocks and try to put the new include such that its order conforms
  to its surrounding.  It's put in the include block which contains
  core kernel includes, in the same order that the rest are ordered -
  alphabetical, Christmas tree, rev-Xmas-tree or at the end if there
  doesn't seem to be any matching order.

* If the script can't find a place to put a new include (mostly
  because the file doesn't have fitting include block), it prints out
  an error message indicating which .h file needs to be added to the
  file.

The conversion was done in the following steps.

1. The initial automatic conversion of all .c files updated slightly
   over 4000 files, deleting around 700 includes and adding ~480 gfp.h
   and ~3000 slab.h inclusions.  The script emitted errors for ~400
   files.

2. Each error was manually checked.  Some didn't need the inclusion,
   some needed manual addition while adding it to implementation .h or
   embedding .c file was more appropriate for others.  This step added
   inclusions to around 150 files.

3. The script was run again and the output was compared to the edits
   from #2 to make sure no file was left behind.

4. Several build tests were done and a couple of problems were fixed.
   e.g. lib/decompress_*.c used malloc/free() wrappers around slab
   APIs requiring slab.h to be added manually.

5. The script was run on all .h files but without automatically
   editing them as sprinkling gfp.h and slab.h inclusions around .h
   files could easily lead to inclusion dependency hell.  Most gfp.h
   inclusion directives were ignored as stuff from gfp.h was usually
   wildly available and often used in preprocessor macros.  Each
   slab.h inclusion directive was examined and added manually as
   necessary.

6. percpu.h was updated not to include slab.h.

7. Build test were done on the following configurations and failures
   were fixed.  CONFIG_GCOV_KERNEL was turned off for all tests (as my
   distributed build env didn't work with gcov compiles) and a few
   more options had to be turned off depending on archs to make things
   build (like ipr on powerpc/64 which failed due to missing writeq).

   * x86 and x86_64 UP and SMP allmodconfig and a custom test config.
   * powerpc and powerpc64 SMP allmodconfig
   * sparc and sparc64 SMP allmodconfig
   * ia64 SMP allmodconfig
   * s390 SMP allmodconfig
   * alpha SMP allmodconfig
   * um on x86_64 SMP allmodconfig

8. percpu.h modifications were reverted so that it could be applied as
   a separate patch and serve as bisection point.

Given the fact that I had only a couple of failures from tests on step
6, I'm fairly confident about the coverage of this conversion patch.
If there is a breakage, it's likely to be something in one of the arch
headers which should be easily discoverable easily on most builds of
the specific arch.

Signed-off-by: Tejun Heo <tj@kernel.org>
Guess-its-ok-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
 2010-03-30 22:02:32 +09:00
David Howells
9ae326a690 CacheFiles: A cache that backs onto a mounted filesystem 
Add an FS-Cache cache-backend that permits a mounted filesystem to be used as a
backing store for the cache.

CacheFiles uses a userspace daemon to do some of the cache management - such as
reaping stale nodes and culling.  This is called cachefilesd and lives in
/sbin.  The source for the daemon can be downloaded from:

	http://people.redhat.com/~dhowells/cachefs/cachefilesd.c

And an example configuration from:

	http://people.redhat.com/~dhowells/cachefs/cachefilesd.conf

The filesystem and data integrity of the cache are only as good as those of the
filesystem providing the backing services.  Note that CacheFiles does not
attempt to journal anything since the journalling interfaces of the various
filesystems are very specific in nature.

CacheFiles creates a misc character device - "/dev/cachefiles" - that is used
to communication with the daemon.  Only one thing may have this open at once,
and whilst it is open, a cache is at least partially in existence.  The daemon
opens this and sends commands down it to control the cache.

CacheFiles is currently limited to a single cache.

CacheFiles attempts to maintain at least a certain percentage of free space on
the filesystem, shrinking the cache by culling the objects it contains to make
space if necessary - see the "Cache Culling" section.  This means it can be
placed on the same medium as a live set of data, and will expand to make use of
spare space and automatically contract when the set of data requires more
space.

============
REQUIREMENTS
============

The use of CacheFiles and its daemon requires the following features to be
available in the system and in the cache filesystem:

	- dnotify.

	- extended attributes (xattrs).

	- openat() and friends.

	- bmap() support on files in the filesystem (FIBMAP ioctl).

	- The use of bmap() to detect a partial page at the end of the file.

It is strongly recommended that the "dir_index" option is enabled on Ext3
filesystems being used as a cache.

=============
CONFIGURATION
=============

The cache is configured by a script in /etc/cachefilesd.conf.  These commands
set up cache ready for use.  The following script commands are available:

 (*) brun <N>%
 (*) bcull <N>%
 (*) bstop <N>%
 (*) frun <N>%
 (*) fcull <N>%
 (*) fstop <N>%

	Configure the culling limits.  Optional.  See the section on culling
	The defaults are 7% (run), 5% (cull) and 1% (stop) respectively.

	The commands beginning with a 'b' are file space (block) limits, those
	beginning with an 'f' are file count limits.

 (*) dir <path>

	Specify the directory containing the root of the cache.  Mandatory.

 (*) tag <name>

	Specify a tag to FS-Cache to use in distinguishing multiple caches.
	Optional.  The default is "CacheFiles".

 (*) debug <mask>

	Specify a numeric bitmask to control debugging in the kernel module.
	Optional.  The default is zero (all off).  The following values can be
	OR'd into the mask to collect various information:

		1	Turn on trace of function entry (_enter() macros)
		2	Turn on trace of function exit (_leave() macros)
		4	Turn on trace of internal debug points (_debug())

	This mask can also be set through sysfs, eg:

		echo 5 >/sys/modules/cachefiles/parameters/debug

==================
STARTING THE CACHE
==================

The cache is started by running the daemon.  The daemon opens the cache device,
configures the cache and tells it to begin caching.  At that point the cache
binds to fscache and the cache becomes live.

The daemon is run as follows:

	/sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]

The flags are:

 (*) -d

	Increase the debugging level.  This can be specified multiple times and
	is cumulative with itself.

 (*) -s

	Send messages to stderr instead of syslog.

 (*) -n

	Don't daemonise and go into background.

 (*) -f <configfile>

	Use an alternative configuration file rather than the default one.

===============
THINGS TO AVOID
===============

Do not mount other things within the cache as this will cause problems.  The
kernel module contains its own very cut-down path walking facility that ignores
mountpoints, but the daemon can't avoid them.

Do not create, rename or unlink files and directories in the cache whilst the
cache is active, as this may cause the state to become uncertain.

Renaming files in the cache might make objects appear to be other objects (the
filename is part of the lookup key).

Do not change or remove the extended attributes attached to cache files by the
cache as this will cause the cache state management to get confused.

Do not create files or directories in the cache, lest the cache get confused or
serve incorrect data.

Do not chmod files in the cache.  The module creates things with minimal
permissions to prevent random users being able to access them directly.

=============
CACHE CULLING
=============

The cache may need culling occasionally to make space.  This involves
discarding objects from the cache that have been used less recently than
anything else.  Culling is based on the access time of data objects.  Empty
directories are culled if not in use.

Cache culling is done on the basis of the percentage of blocks and the
percentage of files available in the underlying filesystem.  There are six
"limits":

 (*) brun
 (*) frun

     If the amount of free space and the number of available files in the cache
     rises above both these limits, then culling is turned off.

 (*) bcull
 (*) fcull

     If the amount of available space or the number of available files in the
     cache falls below either of these limits, then culling is started.

 (*) bstop
 (*) fstop

     If the amount of available space or the number of available files in the
     cache falls below either of these limits, then no further allocation of
     disk space or files is permitted until culling has raised things above
     these limits again.

These must be configured thusly:

	0 <= bstop < bcull < brun < 100
	0 <= fstop < fcull < frun < 100

Note that these are percentages of available space and available files, and do
_not_ appear as 100 minus the percentage displayed by the "df" program.

The userspace daemon scans the cache to build up a table of cullable objects.
These are then culled in least recently used order.  A new scan of the cache is
started as soon as space is made in the table.  Objects will be skipped if
their atimes have changed or if the kernel module says it is still using them.

===============
CACHE STRUCTURE
===============

The CacheFiles module will create two directories in the directory it was
given:

 (*) cache/

 (*) graveyard/

The active cache objects all reside in the first directory.  The CacheFiles
kernel module moves any retired or culled objects that it can't simply unlink
to the graveyard from which the daemon will actually delete them.

The daemon uses dnotify to monitor the graveyard directory, and will delete
anything that appears therein.

The module represents index objects as directories with the filename "I..." or
"J...".  Note that the "cache/" directory is itself a special index.

Data objects are represented as files if they have no children, or directories
if they do.  Their filenames all begin "D..." or "E...".  If represented as a
directory, data objects will have a file in the directory called "data" that
actually holds the data.

Special objects are similar to data objects, except their filenames begin
"S..." or "T...".

If an object has children, then it will be represented as a directory.
Immediately in the representative directory are a collection of directories
named for hash values of the child object keys with an '@' prepended.  Into
this directory, if possible, will be placed the representations of the child
objects:

	INDEX     INDEX      INDEX                             DATA FILES
	========= ========== ================================= ================
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
	cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry

If the key is so long that it exceeds NAME_MAX with the decorations added on to
it, then it will be cut into pieces, the first few of which will be used to
make a nest of directories, and the last one of which will be the objects
inside the last directory.  The names of the intermediate directories will have
'+' prepended:

	J1223/@23/+xy...z/+kl...m/Epqr

Note that keys are raw data, and not only may they exceed NAME_MAX in size,
they may also contain things like '/' and NUL characters, and so they may not
be suitable for turning directly into a filename.

To handle this, CacheFiles will use a suitably printable filename directly and
"base-64" encode ones that aren't directly suitable.  The two versions of
object filenames indicate the encoding:

	OBJECT TYPE	PRINTABLE	ENCODED
	===============	===============	===============
	Index		"I..."		"J..."
	Data		"D..."		"E..."
	Special		"S..."		"T..."

Intermediate directories are always "@" or "+" as appropriate.

Each object in the cache has an extended attribute label that holds the object
type ID (required to distinguish special objects) and the auxiliary data from
the netfs.  The latter is used to detect stale objects in the cache and update
or retire them.

Note that CacheFiles will erase from the cache any file it doesn't recognise or
any file of an incorrect type (such as a FIFO file or a device file).

==========================
SECURITY MODEL AND SELINUX
==========================

CacheFiles is implemented to deal properly with the LSM security features of
the Linux kernel and the SELinux facility.

One of the problems that CacheFiles faces is that it is generally acting on
behalf of a process, and running in that process's context, and that includes a
security context that is not appropriate for accessing the cache - either
because the files in the cache are inaccessible to that process, or because if
the process creates a file in the cache, that file may be inaccessible to other
processes.

The way CacheFiles works is to temporarily change the security context (fsuid,
fsgid and actor security label) that the process acts as - without changing the
security context of the process when it the target of an operation performed by
some other process (so signalling and suchlike still work correctly).

When the CacheFiles module is asked to bind to its cache, it:

 (1) Finds the security label attached to the root cache directory and uses
     that as the security label with which it will create files.  By default,
     this is:

	cachefiles_var_t

 (2) Finds the security label of the process which issued the bind request
     (presumed to be the cachefilesd daemon), which by default will be:

	cachefilesd_t

     and asks LSM to supply a security ID as which it should act given the
     daemon's label.  By default, this will be:

	cachefiles_kernel_t

     SELinux transitions the daemon's security ID to the module's security ID
     based on a rule of this form in the policy.

	type_transition <daemon's-ID> kernel_t : process <module's-ID>;

     For instance:

	type_transition cachefilesd_t kernel_t : process cachefiles_kernel_t;

The module's security ID gives it permission to create, move and remove files
and directories in the cache, to find and access directories and files in the
cache, to set and access extended attributes on cache objects, and to read and
write files in the cache.

The daemon's security ID gives it only a very restricted set of permissions: it
may scan directories, stat files and erase files and directories.  It may
not read or write files in the cache, and so it is precluded from accessing the
data cached therein; nor is it permitted to create new files in the cache.

There are policy source files available in:

	http://people.redhat.com/~dhowells/fscache/cachefilesd-0.8.tar.bz2

and later versions.  In that tarball, see the files:

	cachefilesd.te
	cachefilesd.fc
	cachefilesd.if

They are built and installed directly by the RPM.

If a non-RPM based system is being used, then copy the above files to their own
directory and run:

	make -f /usr/share/selinux/devel/Makefile
	semodule -i cachefilesd.pp

You will need checkpolicy and selinux-policy-devel installed prior to the
build.

By default, the cache is located in /var/fscache, but if it is desirable that
it should be elsewhere, than either the above policy files must be altered, or
an auxiliary policy must be installed to label the alternate location of the
cache.

For instructions on how to add an auxiliary policy to enable the cache to be
located elsewhere when SELinux is in enforcing mode, please see:

	/usr/share/doc/cachefilesd-*/move-cache.txt

When the cachefilesd rpm is installed; alternatively, the document can be found
in the sources.

==================
A NOTE ON SECURITY
==================

CacheFiles makes use of the split security in the task_struct.  It allocates
its own task_security structure, and redirects current->act_as to point to it
when it acts on behalf of another process, in that process's context.

The reason it does this is that it calls vfs_mkdir() and suchlike rather than
bypassing security and calling inode ops directly.  Therefore the VFS and LSM
may deny the CacheFiles access to the cache data because under some
circumstances the caching code is running in the security context of whatever
process issued the original syscall on the netfs.

Furthermore, should CacheFiles create a file or directory, the security
parameters with that object is created (UID, GID, security label) would be
derived from that process that issued the system call, thus potentially
preventing other processes from accessing the cache - including CacheFiles's
cache management daemon (cachefilesd).

What is required is to temporarily override the security of the process that
issued the system call.  We can't, however, just do an in-place change of the
security data as that affects the process as an object, not just as a subject.
This means it may lose signals or ptrace events for example, and affects what
the process looks like in /proc.

So CacheFiles makes use of a logical split in the security between the
objective security (task->sec) and the subjective security (task->act_as).  The
objective security holds the intrinsic security properties of a process and is
never overridden.  This is what appears in /proc, and is what is used when a
process is the target of an operation by some other process (SIGKILL for
example).

The subjective security holds the active security properties of a process, and
may be overridden.  This is not seen externally, and is used whan a process
acts upon another object, for example SIGKILLing another process or opening a
file.

LSM hooks exist that allow SELinux (or Smack or whatever) to reject a request
for CacheFiles to run in a context of a specific security label, or to create
files and directories with another security label.

This documentation is added by the patch to:

	Documentation/filesystems/caching/cachefiles.txt

Signed-Off-By: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
 2009-04-03 16:42:41 +01:00