Re: [PATCH AUTOSEL 5.7 244/274] xfs: force writes to delalloc regions to unwritten

Darrick J. Wong
2020-06-09T01:07:44Z

On Mon, Jun 08, 2020 at 07:05:37PM -0400, Sasha Levin wrote:
> From: "Darrick J. Wong" <darrick.wong@oracle.com>
>
> [ Upstream commit a5949d3faedf492fa7863b914da408047ab46eb0 ]
>
> When writing to a delalloc region in the data fork, commit the new
> allocations (of the da reservation) as unwritten so that the mappings
> are only marked written once writeback completes successfully. This
> fixes the problem of stale data exposure if the system goes down during
> targeted writeback of a specific region of a file, as tested by
> generic/042.
>
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

Err, this doesn't have a Fixes: tag attached to it. Does it pass
fstests? Because it doesn't look like you've pulled in "xfs: don't fail
unwritten extent conversion on writeback due to edquot", which is needed
to avoid regressing fstests...

...waitaminute, that whole series lacks Fixes: tags because it wasn't
considered a good enough candidate for automatic backport.

Ummm, does the autosel fstests driver turn on quotas? ;)

--D

> ---
> fs/xfs/libxfs/xfs_bmap.c | 29 +++++++++++++++++------------
> 1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index fda13cd7add0..f8fe83c9348d 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -4193,17 +4193,7 @@ xfs_bmapi_allocate(
> bma->got.br_blockcount = bma->length;
> bma->got.br_state = XFS_EXT_NORM;
>
> - /*
> - * In the data fork, a wasdelay extent has been initialized, so
> - * shouldn't be flagged as unwritten.
> - *
> - * For the cow fork, however, we convert delalloc reservations
> - * (extents allocated for speculative preallocation) to
> - * allocated unwritten extents, and only convert the unwritten
> - * extents to real extents when we're about to write the data.
> - */
> - if ((!bma->wasdel || (bma->flags & XFS_BMAPI_COWFORK)) &&
> - (bma->flags & XFS_BMAPI_PREALLOC))
> + if (bma->flags & XFS_BMAPI_PREALLOC)
> bma->got.br_state = XFS_EXT_UNWRITTEN;
>
> if (bma->wasdel)
> @@ -4611,8 +4601,23 @@ xfs_bmapi_convert_delalloc(
> bma.offset = bma.got.br_startoff;
> bma.length = max_t(xfs_filblks_t, bma.got.br_blockcount, MAXEXTLEN);
> bma.minleft = xfs_bmapi_minleft(tp, ip, whichfork);
> +
> + /*
> + * When we're converting the delalloc reservations backing dirty pages
> + * in the page cache, we must be careful about how we create the new
> + * extents:
> + *
> + * New CoW fork extents are created unwritten, turned into real extents
> + * when we're about to write the data to disk, and mapped into the data
> + * fork after the write finishes. End of story.
> + *
> + * New data fork extents must be mapped in as unwritten and converted
> + * to real extents after the write succeeds to avoid exposing stale
> + * disk contents if we crash.
> + */
> + bma.flags = XFS_BMAPI_PREALLOC;
> if (whichfork == XFS_COW_FORK)
> - bma.flags = XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC;
> + bma.flags |= XFS_BMAPI_COWFORK;
>
> if (!xfs_iext_peek_prev_extent(ifp, &bma.icur, &bma.prev))
> bma.prev.br_startoff = NULLFILEOFF;
> --
> 2.25.1
>