Re: [PATCH v23.2 3/4] xfs: log the AGI/AGF buffers when rolling transactions during an AG repair

[Dave Chinner <david@fromorbit.com>]

On Mon, Oct 31, 2022 at 11:08:33AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Currently, the only way to lock an allocation group is to hold the AGI
> and AGF buffers.  If a repair needs to roll the transaction while
> repairing some AG metadata, it maintains that lock by holding the two
> buffers across the transaction roll and joins them afterwards.
> 
> However, repair is not like other parts of XFS that employ the bhold -
> roll - bjoin sequence because it's possible that the AGI or AGF buffers
> are not actually dirty before the roll.  This presents two problems --
> First, we need to redirty those buffers to keep them moving along in the
> log to avoid pinning the log tail.  Second, a clean buffer log item can
> detach from the buffer.  If this happens, the buffer type state is
> discarded along with the bli and must be reattached before the next time
> the buffer is logged.   If it is not, the logging code will complain and
> log recovery will not work properly.
> 
> An earlier version of this patch tried to fix the second problem by
> re-setting the buffer type in the bli after joining the buffer to the
> new transaction, but that looked weird and didn't solve the first
> problem.  Instead, solve both problems by logging the buffer before
> rolling the transaction.
> 
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>

I guess this is fine as long as it is confined to the scrub code;
if we need to hold clean buffers across transaction rolls in other
code we really need to sort out the BLI life cycle issues that this
currently exposes.

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