xfs: xfs_reflink_convert_cow() memory allocation deadlock

xfs_reflink_convert_cow() manipulates the incore extent list
in GFP_KERNEL context in the IO submission path whilst holding
locked pages under writeback. This is a memory reclaim deadlock
vector. This code is not in a transaction, so any memory allocations
it makes aren't protected via the memalloc_nofs_save() context that
transactions carry.

Hence we need to run this call under memalloc_nofs_save() context to
prevent potential memory allocations from being run as GFP_KERNEL
and deadlocking.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_aops.c

@@ -531,8 +531,19 @@ xfs_submit_ioend(
 {
 	/* Convert CoW extents to regular */
 	if (!status && ioend->io_type == XFS_IO_COW) {
+		/*
+		 * Yuk. This can do memory allocation, but is not a
+		 * transactional operation so everything is done in GFP_KERNEL
+		 * context. That can deadlock, because we hold pages in
+		 * writeback state and GFP_KERNEL allocations can block on them.
+		 * Hence we must operate in nofs conditions here.
+		 */
+		unsigned nofs_flag;
+
+		nofs_flag = memalloc_nofs_save();
 		status = xfs_reflink_convert_cow(XFS_I(ioend->io_inode),
 				ioend->io_offset, ioend->io_size);
+		memalloc_nofs_restore(nofs_flag);
 	}
 
 	/* Reserve log space if we might write beyond the on-disk inode size. */

fs/xfs/xfs_buf.c

@@ -21,7 +21,6 @@
 #include <linux/migrate.h>
 #include <linux/backing-dev.h>
 #include <linux/freezer.h>
-#include <linux/sched/mm.h>
 
 #include "xfs_format.h"
 #include "xfs_log_format.h"

fs/xfs/xfs_linux.h

@@ -26,6 +26,7 @@ typedef __u32			xfs_nlink_t;
 
 #include <linux/semaphore.h>
 #include <linux/mm.h>
+#include <linux/sched/mm.h>
 #include <linux/kernel.h>
 #include <linux/blkdev.h>
 #include <linux/slab.h>