Re: [PATCH 2/2] xfs: estimate post-merge refcounts correctly

[Dave Chinner <david@fromorbit.com>]

On Tue, Nov 29, 2022 at 02:01:36PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Upon enabling fsdax + reflink for XFS, xfs/179 began to report refcount
> metadata corruptions after being run.  Specifically, xfs_repair noticed
> single-block refcount records that could be combined but had not been.
> 
> The root cause of this is improper MAXREFCOUNT edge case handling in
> xfs_refcount_merge_extents.  When we're trying to find candidates for a
> refcount btree record merge, we compute the refcount attribute of the
> merged record, but we fail to account for the fact that once a record
> hits rc_refcount == MAXREFCOUNT, it is pinned that way forever.  Hence
> the computed refcount is wrong, and we fail to merge the extents.
> 
> Fix this by adjusting the merge predicates to compute the adjusted
> refcount correctly.
> 
> Fixes: 3172725814f9 ("xfs: adjust refcount of an extent of blocks in refcount btree")
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> ---
>  fs/xfs/libxfs/xfs_refcount.c |   25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)

Looks good to me.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

-- 
Dave Chinner
david@fromorbit.com