Re: [PATCH v3] xfs: clear PF_MEMALLOC before exiting xfsaild thread

Darrick J. Wong
2020-03-10T15:47:08Z

On Mon, Mar 09, 2020 at 11:57:14AM -0700, Eric Biggers wrote:
> 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>

Seems reasonable to me, will give it a spin...
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>
> 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
>