[PATCH 09/19] XFS: ensure xfs_file_*_read cannot deadlock in memory allocation.

NeilBrown
2014-04-16T04:18:58Z

xfs_file_*_read holds an inode lock while calling a generic 'read'
function. These functions perform read-ahead and are quite likely to
allocate memory.
So set PF_FSTRANS to ensure they avoid __GFP_FS and so don't recurse
into a filesystem to free memory.

This can be a problem with loop-back NFS mounts, if free_pages ends up
wating in nfs_release_page(), and nfsd is blocked waiting for the lock
that this code holds.

This was found both by lockdep and as a real deadlock during testing.

Signed-off-by: NeilBrown <neilb@suse.de>
---
fs/xfs/xfs_file.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 64b48eade91d..88b33ef64668 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -243,6 +243,7 @@ xfs_file_aio_read(
ssize_t ret = 0;
int ioflags = 0;
xfs_fsize_t n;
+ unsigned int pflags;

XFS_STATS_INC(xs_read_calls);

@@ -290,6 +291,10 @@ xfs_file_aio_read(
* proceeed concurrently without serialisation.
*/
xfs_rw_ilock(ip, XFS_IOLOCK_SHARED);
+ /* As we hold a lock, we must ensure that any allocation
+ * in generic_file_aio_read avoid __GFP_FS
+ */
+ current_set_flags_nested(&pflags, PF_FSTRANS);
if ((ioflags & IO_ISDIRECT) && inode->i_mapping->nrpages) {
xfs_rw_iunlock(ip, XFS_IOLOCK_SHARED);
xfs_rw_ilock(ip, XFS_IOLOCK_EXCL);
@@ -313,6 +318,7 @@ xfs_file_aio_read(
if (ret > 0)
XFS_STATS_ADD(xs_read_bytes, ret);

+ current_restore_flags_nested(&pflags, PF_FSTRANS);
xfs_rw_iunlock(ip, XFS_IOLOCK_SHARED);
return ret;
}
@@ -328,6 +334,7 @@ xfs_file_splice_read(
struct xfs_inode *ip = XFS_I(infilp->f_mapping->host);
int ioflags = 0;
ssize_t ret;
+ unsigned int pflags;

XFS_STATS_INC(xs_read_calls);

@@ -338,6 +345,10 @@ xfs_file_splice_read(
return -EIO;

xfs_rw_ilock(ip, XFS_IOLOCK_SHARED);
+ /* As we hold a lock, we must ensure that any allocation
+ * in generic_file_splice_read avoid __GFP_FS
+ */
+ current_set_flags_nested(&pflags, PF_FSTRANS);

trace_xfs_file_splice_read(ip, count, *ppos, ioflags);

@@ -345,6 +356,7 @@ xfs_file_splice_read(
if (ret > 0)
XFS_STATS_ADD(xs_read_bytes, ret);

+ current_restore_flags_nested(&pflags, PF_FSTRANS);
xfs_rw_iunlock(ip, XFS_IOLOCK_SHARED);
return ret;
}