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>

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 767d532..1eb625f 100644 (file)
--- a/fs/xfs/xfs_aops.c
+++ b/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. */

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 980bc48..e9c058e 100644 (file)
--- a/fs/xfs/xfs_buf.c
+++ b/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"

diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index ae1e66f..1631cf4 100644 (file)
--- a/fs/xfs/xfs_linux.h
+++ b/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>