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

["Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com>]

On 2022/11/30 6:01, 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.
Hi Darrick,

I am investigating the issue as well. Thanks a lot for your quick fix.
I have confirmed that xfs/179 with the fix patch can works well in DAX mode.
Reviewed-by: Xiao Yang <yangx.jy@fujitsu.com>

> 
> 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

One question:
Is it reansonable/expected to pin rc_refcount forever when a record hits 
rc_refcount == MAXREFCOUNT?

> 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.

Best Regards,
Xiao Yang