[PATCH for-4.2 v2 0/3] qcow2: Fix data corruption on XFS

From: Max Reitz <mreitz@redhat.com>
To: qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
	Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>,
	qemu-stable@nongnu.org, qemu-devel@nongnu.org,
	Anton Nefedov <anton.nefedov@virtuozzo.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	"Denis V . Lunev" <den@openvz.org>, Max Reitz <mreitz@redhat.com>
Subject: [PATCH for-4.2 v2 0/3] qcow2: Fix data corruption on XFS
Date: Fri,  1 Nov 2019 16:25:07 +0100	[thread overview]
Message-ID: <20191101152510.11719-1-mreitz@redhat.com> (raw)

Hi,

As I reasoned here:
https://lists.nongnu.org/archive/html/qemu-block/2019-11/msg00027.html
I’m no longer convinced of reverting c8bb23cbdbe.  I could see a
significant performance improvement from it on ext4 with aio=native in a
guest that does random 4k writes, and as such I feel it would be a
regression to revert it for 4.2.

To work around the XFS corruption, we still need the other three patches
from the series, of course.  We cannot restrict the workaround to just
XFS, because maybe the file is on a remote filesystem and then we don’t
know anything about the host configuration.

The performance impact should still be minimal because this just
serializes post-EOF write-zeroes and data writes, and that just doesn’t
happen very often, even with handle_alloc_space() in qcow2.

I would love to have more time to make a decision, but there simply
isn’t any.  Patches for 4.1.1 are to be collected on Monday, AFAIU.

v2:
- Dropped patch 1
- Forgot a “coroutine_fn” in patch 2 (it isn’t really important,
  qemu_coroutine_self() works in non-coroutine functions, too, but
  calling bdrv_(co_)get_self_request() from a non-coroutine just doesn’t
  make any sense)
- Patch 3:
  - Reverted the commit message to the RFC state to reflect that this is
    specifically because of handle_alloc_space()
  - Dropped the two lines that said there’d be no performance impact at
    all because no driver would submit zero writes beyond the EOF
    (because qcow2 now still does)

git-backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/3:[----] [--] 'block: Make wait/mark serialising requests public'
002/3:[0004] [FC] 'block: Add bdrv_co_get_self_request()'
003/3:[0002] [FC] 'block/file-posix: Let post-EOF fallocate serialize'

Max Reitz (3):
  block: Make wait/mark serialising requests public
  block: Add bdrv_co_get_self_request()
  block/file-posix: Let post-EOF fallocate serialize

 include/block/block_int.h |  4 ++++
 block/file-posix.c        | 36 +++++++++++++++++++++++++++++++++
 block/io.c                | 42 ++++++++++++++++++++++++++++-----------
 3 files changed, 70 insertions(+), 12 deletions(-)

-- 
2.21.0