Re: [PATCH] xfs: trim writepage mapping to within eof

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 12, 2017 at 07:47:54AM -0400, Brian Foster wrote:
> The writeback rework in commit fbcc02561359 ("xfs: Introduce
> writeback context for writepages") introduced a subtle change in
> behavior with regard to the block mapping used across the
> ->writepages() sequence. The previous xfs_cluster_write() code would
> only flush pages up to EOF at the time of the writepage, thus
> ensuring that any pages due to file-extending writes would be
> handled on a separate cycle and with a new, updated block mapping.
> 
> The updated code establishes a block mapping in xfs_writepage_map()
> that could extend beyond EOF if the file has post-eof preallocation.
> Because we now use the generic writeback infrastructure and pass the
> cached mapping to each writepage call, there is no implicit EOF
> limit in place. If eofblocks trimming occurs during ->writepages(),
> any post-eof portion of the cached mapping becomes invalid. The
> eofblocks code has no means to serialize against writeback because
> there are no pages associated with post-eof blocks. Therefore if an
> eofblocks trim occurs and is followed by a file-extending buffered
> write, not only has the mapping become invalid, but we could end up
> writing a page to disk based on the invalid mapping.
> 
> Consider the following sequence of events:
> 
> - A buffered write creates a delalloc extent and post-eof
>   speculative preallocation.
> - Writeback starts and on the first writepage cycle, the delalloc
>   extent is converted to real blocks (including the post-eof blocks)
>   and the mapping is cached.
> - The file is closed and xfs_release() trims post-eof blocks. The
>   cached writeback mapping is now invalid.
> - Another buffered write appends the file with a delalloc extent.
> - The concurrent writeback cycle picks up the just written page
>   because the writeback range end is LLONG_MAX. xfs_writepage_map()
>   attributes it to the (now invalid) cached mapping and writes the
>   data to an incorrect location on disk (and where the file offset is
>   still backed by a delalloc extent).
> 
> This problem is reproduced by xfstests test generic/463, which
> triggers racing writes, appends, open/closes and writeback requests.
> 
> To address this problem, trim the mapping used during writeback to
> within EOF when the mapping is created. This ensures the mapping is
> revalidated for any pages encountered beyond EOF as of the time the
> current mapping was cached.
> 
> Reported-by: Eryu Guan <eguan@redhat.com>
> Diagnosed-by: Eryu Guan <eguan@redhat.com>
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
> 
> Hi all,
> 
> This is a followup to the issue Eryu tracked down, described here[1].
> 
> Note that this patch will not deal with any writeback mapping validity
> issues not associated with eofblocks management. Dave is working on a
> more generic approach to deal with such problems. This patch is intended
> to be a targeted and backportable fix for the regression in the
> writeback code.
> 
> Brian
> 
> [1] https://marc.info/?l=linux-xfs&m=150406724427829&w=2
> 
>  fs/xfs/libxfs/xfs_bmap.c | 11 +++++++++++
>  fs/xfs/libxfs/xfs_bmap.h |  1 +
>  fs/xfs/xfs_aops.c        |  6 ++++--
>  3 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 044a363..dd3fb7b 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3852,6 +3852,17 @@ xfs_trim_extent(
>  	}
>  }
>  
> +/* trim extent to within eof */
> +void
> +xfs_trim_extent_eof(
> +	struct xfs_bmbt_irec	*irec,
> +	struct xfs_inode	*ip)
> +
> +{
> +	xfs_trim_extent(irec, 0, XFS_B_TO_FSB(ip->i_mount,
> +					      i_size_read(VFS_I(ip))));
> +}

Ok, so it's an unlocked, instantaneous sample of the inode size.
Truncate can race with this and still occur any time after we've
trimmed the extent but still have it cached.

As such, I'm thinking this EOF trimming it should be put in
xfs_imap_valid() - not xfs_map_blocks() - so it gets revalidated
every time we check to see if the map covers the current file
extent...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com