Re: [PATCH 3/3] xfs: use a separate frextents counter for rt extent reservations

[Dave Chinner <david@fromorbit.com>]

On Sun, Apr 10, 2022 at 11:21:11AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> As mentioned in the previous commit, the kernel misuses sb_frextents in
> the incore mount to reflect both incore reservations made by running
> transactions as well as the actual count of free rt extents on disk.
> This results in the superblock being written to the log with an
> underestimate of the number of rt extents that are marked free in the
> rtbitmap.
> 
> Teaching XFS to recompute frextents after log recovery avoids
> operational problems in the current mount, but it doesn't solve the
> problem of us writing undercounted frextents which are then recovered by
> an older kernel that doesn't have that fix.
> 
> Create an incore percpu counter to mirror the ondisk frextents.  This
> new counter will track transaction reservations and the only time we
> will touch the incore super counter (i.e the one that gets logged) is
> when those transactions commit updates to the rt bitmap.  This is in
> contrast to the lazysbcount counters (e.g. fdblocks), where we know that
> log recovery will always fix any incorrect counter that we log.
> As a bonus, we only take m_sb_lock at transaction commit time.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Looks good - much neater than th first version and I really like the
way you did the xfs_mod_freecounter() factoring.

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