Darrick,
can we get this queue up for 5.0?
On Wed, Jan 23, 2019 at 01:41:26PM -0500, Brian Foster wrote:
> The cached writeback mapping is EOF trimmed to try and avoid races
> between post-eof block management and writeback that result in
> sending cached data to a stale location. The cached mapping is
> currently trimmed on the validation check, which leaves a race
> window between the time the mapping is cached and when it is trimmed
> against the current inode size.
>
> For example, if a new mapping is cached by delalloc conversion on a
> blocksize == page size fs, we could cycle various locks, perform
> memory allocations, etc. in the writeback codepath before the
> associated mapping is eventually trimmed to i_size. This leaves
> enough time for a post-eof truncate and file append before the
> cached mapping is trimmed. The former event essentially invalidates
> a range of the cached mapping and the latter bumps the inode size
> such the trim on the next writepage event won't trim all of the
> invalid blocks. fstest generic/464 reproduces this scenario
> occasionally and causes a lost writeback and stale delalloc blocks
> warning on inode inactivation.
>
> To work around this problem, trim the cached writeback mapping as
> soon as it is cached in addition to on subsequent validation checks.
> This is a minor tweak to tighten the race window as much as possible
> until a proper invalidation mechanism is available.
>
> Fixes: 40214d128e07 ("xfs: trim writepage mapping to within eof")
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.14+
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> Reviewed-by: Allison Henderson <allison.henderson@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
> fs/xfs/xfs_aops.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index 338b9d9984e0..d9048bcea49c 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -449,6 +449,7 @@ xfs_map_blocks(
> }
>
> wpc->imap = imap;
> + xfs_trim_extent_eof(&wpc->imap, ip);
> trace_xfs_map_blocks_found(ip, offset, count, wpc->io_type, &imap);
> return 0;
> allocate_blocks:
> @@ -459,6 +460,7 @@ xfs_map_blocks(
> ASSERT(whichfork == XFS_COW_FORK || cow_fsb == NULLFILEOFF ||
> imap.br_startoff + imap.br_blockcount <= cow_fsb);
> wpc->imap = imap;
> + xfs_trim_extent_eof(&wpc->imap, ip);
> trace_xfs_map_blocks_alloc(ip, offset, count, wpc->io_type, &imap);
> return 0;
> }
> --
> 2.17.2
>
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