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

[Brian Foster <bfoster@redhat.com>]

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

Reviewed-by: Brian Foster <bfoster@redhat.com>

> 
> 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
>