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Subject[PATCH v3] xfs: clear PF_MEMALLOC before exiting xfsaild thread
Date
From: Eric Biggers <ebiggers@google.com>

Leaving PF_MEMALLOC set when exiting a kthread causes it to remain set
during do_exit(). That can confuse things. In particular, if BSD
process accounting is enabled, then do_exit() writes data to an
accounting file. If that file has FS_SYNC_FL set, then this write
occurs synchronously and can misbehave if PF_MEMALLOC is set.

For example, if the accounting file is located on an XFS filesystem,
then a WARN_ON_ONCE() in iomap_do_writepage() is triggered and the data
doesn't get written when it should. Or if the accounting file is
located on an ext4 filesystem without a journal, then a WARN_ON_ONCE()
in ext4_write_inode() is triggered and the inode doesn't get written.

Fix this in xfsaild() by using the helper functions to save and restore
PF_MEMALLOC.

This can be reproduced as follows in the kvm-xfstests test appliance
modified to add the 'acct' Debian package, and with kvm-xfstests's
recommended kconfig modified to add CONFIG_BSD_PROCESS_ACCT=y:

mkfs.xfs -f /dev/vdb
mount /vdb
touch /vdb/file
chattr +S /vdb/file
accton /vdb/file
mkfs.xfs -f /dev/vdc
mount /vdc
umount /vdc

It causes:
WARNING: CPU: 1 PID: 336 at fs/iomap/buffered-io.c:1534
CPU: 1 PID: 336 Comm: xfsaild/vdc Not tainted 5.6.0-rc5 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20191223_100556-anatol 04/01/2014
RIP: 0010:iomap_do_writepage+0x16b/0x1f0 fs/iomap/buffered-io.c:1534
[...]
Call Trace:
write_cache_pages+0x189/0x4d0 mm/page-writeback.c:2238
iomap_writepages+0x1c/0x33 fs/iomap/buffered-io.c:1642
xfs_vm_writepages+0x65/0x90 fs/xfs/xfs_aops.c:578
do_writepages+0x41/0xe0 mm/page-writeback.c:2344
__filemap_fdatawrite_range+0xd2/0x120 mm/filemap.c:421
file_write_and_wait_range+0x71/0xc0 mm/filemap.c:760
xfs_file_fsync+0x7a/0x2b0 fs/xfs/xfs_file.c:114
generic_write_sync include/linux/fs.h:2867 [inline]
xfs_file_buffered_aio_write+0x379/0x3b0 fs/xfs/xfs_file.c:691
call_write_iter include/linux/fs.h:1901 [inline]
new_sync_write+0x130/0x1d0 fs/read_write.c:483
__kernel_write+0x54/0xe0 fs/read_write.c:515
do_acct_process+0x122/0x170 kernel/acct.c:522
slow_acct_process kernel/acct.c:581 [inline]
acct_process+0x1d4/0x27c kernel/acct.c:607
do_exit+0x83d/0xbc0 kernel/exit.c:791
kthread+0xf1/0x140 kernel/kthread.c:257
ret_from_fork+0x27/0x50 arch/x86/entry/entry_64.S:352

This bug was originally reported by syzbot at
https://lore.kernel.org/r/0000000000000e7156059f751d7b@google.com.

Reported-by: syzbot+1f9dc49e8de2582d90c2@syzkaller.appspotmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
---

v3: updated commit message again, this time to take into account the bug
also being reproducible when the accounting file is located on XFS.

v2: include more details in the commit message.

fs/xfs/xfs_trans_ail.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
index 00cc5b8734be8..3bc570c90ad97 100644
--- a/fs/xfs/xfs_trans_ail.c
+++ b/fs/xfs/xfs_trans_ail.c
@@ -529,8 +529,9 @@ xfsaild(
{
struct xfs_ail *ailp = data;
long tout = 0; /* milliseconds */
+ unsigned int noreclaim_flag;

- current->flags |= PF_MEMALLOC;
+ noreclaim_flag = memalloc_noreclaim_save();
set_freezable();

while (1) {
@@ -601,6 +602,7 @@ xfsaild(
tout = xfsaild_push(ailp);
}

+ memalloc_noreclaim_restore(noreclaim_flag);
return 0;
}

--
2.25.1
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