Re: [PATCH v4 for-6.0? 0/3] qcow2: fix parallel rewrite and discard (rw-lock)

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

ping. Do we want it for 6.0?

19.03.2021 13:08, Vladimir Sementsov-Ogievskiy wrote:
> Look at 03 for the problem and fix. 01 is preparation and 02 is the
> test.
> 
> Actually previous version of this thing is
>     [PATCH v2(RFC) 0/3] qcow2: fix parallel rewrite and discard
> 
> Still
>     [PATCH v3 0/6] qcow2: compressed write cache
> includes another fix (more complicated) for the bug, so this is called
> v4.
> 
> So, what's new:
> 
> It's still a CoRwlock based solution as suggested by Kevin.
> 
> Now I think that "writer" of the lock should be code in
> update_refcount() which wants to set refcount to zero. If we consider
> only guest discard request as "writer" we may miss other sources of
> discarding host clusters (like rewriting compressed cluster to normal,
> maybe some snapshot operations, who knows what's more).
> 
> And this means that we want to take rw-lock under qcow2 s->lock. And
> this brings ordering restriction for the two locks: if we want both
> locks taken, we should always take s->lock first, and never take s->lock
> when rw-lock is already taken (otherwise we get classic deadlock).
> 
> This leads us to taking rd-lock for in-flight writes under s->lock in
> same critical section where cluster is allocated (or just got from
> metadata) and releasing after data writing completion.
> 
> This in turn leads to a bit tricky logic around transferring rd-lock to
> task coroutine on normal write path (see 03).. But this is still simpler
> than inflight-write-counters solution in v3..
> 
> Vladimir Sementsov-Ogievskiy (3):
>    qemu-io: add aio_discard
>    iotests: add qcow2-discard-during-rewrite
>    block/qcow2: introduce discard_rw_lock: fix discarding host clusters
> 
>   block/qcow2.h                                 |  20 +++
>   block/qcow2-refcount.c                        |  22 ++++
>   block/qcow2.c                                 |  73 +++++++++--
>   qemu-io-cmds.c                                | 117 ++++++++++++++++++
>   .../tests/qcow2-discard-during-rewrite        |  99 +++++++++++++++
>   .../tests/qcow2-discard-during-rewrite.out    |  17 +++
>   6 files changed, 341 insertions(+), 7 deletions(-)
>   create mode 100755 tests/qemu-iotests/tests/qcow2-discard-during-rewrite
>   create mode 100644 tests/qemu-iotests/tests/qcow2-discard-during-rewrite.out
> 


-- 
Best regards,
Vladimir