Re: [PATCHSET v26.0 0/9] xfs: fix online repair block reaping

Dave Chinner
2023-08-07T06:19:19Z

On Thu, Jul 27, 2023 at 03:18:32PM -0700, Darrick J. Wong wrote:
> Hi all,
>
> These patches fix a few problems that I noticed in the code that deals
> with old btree blocks after a successful repair.
>
> First, I observed that it is possible for repair to incorrectly
> invalidate and delete old btree blocks if they were crosslinked. The
> solution here is to consult the reverse mappings for each block in the
> extent -- singly owned blocks are invalidated and freed, whereas for
> crosslinked blocks, we merely drop the incorrect reverse mapping.
>
> A largeish change in this patchset is moving the reaping code to a
> separate file, because the code are mostly interrelated static
> functions. For now this also drops the ability to reap file blocks,
> which will return when we add the bmbt repair functions.
>
> Second, we convert the reap function to use EFIs so that we can commit
> to freeing as many blocks in as few transactions as we dare. We would
> like to free as many old blocks as we can in the same transaction that
> commits the new structure to the ondisk filesystem to minimize the
> number of blocks that leak if the system crashes before the repair fully
> completes.
>
> The third change made in this series is to avoid tripping buffer cache
> assertions if we're merely scanning the buffer cache for buffers to
> invalidate, and find a non-stale buffer of the wrong length. This is
> primarily cosmetic, but makes my life easier.
>
> The fourth change restructures the reaping code to try to process as many
> blocks in one go as possible, to reduce logging traffic.
>
> The last change switches the reaping mechanism to use per-AG bitmaps
> defined in a previous patchset. This should reduce type confusion when
> reading the source code.
>
> If you're going to start using this mess, you probably ought to just
> pull from my git trees, which are linked below.
>
> This is an extraordinary way to destroy everything. Enjoy!
> Comments and questions are, as always, welcome.

Overall I don't see any red flags, so from that perspective I think
it's good to merge as is. THe buffer cache interactions are much
neater this time around.

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

The main thing I noticed is that the deferred freeing mechanism ifo
rbulk reaping will add up to 128 XEFIs to the transaction. That
could result in a single EFI with up to 128 extents in it, right?

What happens when we try to free that many extents in a single
transaction loop? The extent free processing doesn't have a "have we
run out of transaction reservation" check in it like the refcount
item processing does, so I don't think it can roll to renew the
transaction reservation if it is needed. DO we need to catch this
and renew the reservation by returning -EAGAIN from
xfs_extent_free_finish_item() if there isn't enough of a reservation
remaining to free an extent?

-Dave.
--
Dave Chinner
david@fromorbit.com