Re: [PATCH v2] xfs: clear PF_MEMALLOC before exiting xfsaild thread

Darrick J. Wong
2020-03-09T16:24:46Z

On Sun, Mar 08, 2020 at 09:34:30PM -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. For example, if BSD process
> accounting is enabled and the accounting file has FS_SYNC_FL set and is
> located on an ext4 filesystem without a journal, then do_exit() ends up
> calling ext4_write_inode(). That triggers the
> WARN_ON_ONCE(current->flags & PF_MEMALLOC) there, as it assumes
> (appropriately) that inodes aren't written when allocating memory.
>
> 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.ext2 -F /dev/vdb
> mount /vdb -t ext4
> touch /vdb/file
> chattr +S /vdb/file

Does this trip if the process accounting file is also on an xfs
filesystem?

> accton /vdb/file
> mkfs.xfs -f /dev/vdc
> mount /vdc
> umount /vdc

...and if so, can this be turned into an fstests case, please?

>
> It causes:
> WARNING: CPU: 0 PID: 332 at fs/ext4/inode.c:5097 ext4_write_inode+0x140/0x1a0
> CPU: 0 PID: 332 Comm: xfsaild/vdc Not tainted 5.6.0-rc5 #5
> [...]
> RIP: 0010:ext4_write_inode+0x140/0x1a0 fs/ext4/inode.c:5097
> [...]
> Call Trace:
> write_inode fs/fs-writeback.c:1312 [inline]
> __writeback_single_inode+0x465/0x5f0 fs/fs-writeback.c:1511
> writeback_single_inode+0xad/0x120 fs/fs-writeback.c:1565
> sync_inode fs/fs-writeback.c:2602 [inline]
> sync_inode_metadata+0x3d/0x57 fs/fs-writeback.c:2622
> ext4_fsync_nojournal fs/ext4/fsync.c:94 [inline]
> ext4_sync_file+0x243/0x4b0 fs/ext4/fsync.c:172
> generic_write_sync include/linux/fs.h:2867 [inline]
> ext4_buffered_write_iter+0xe1/0x130 fs/ext4/file.c:277
> 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 case 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>

Looks ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

> ---
>
> 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
>