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Lunev" Cc: "Denis V. Lunev" , qemu-devel@nongnu.org, qemu-block@nongnu.org, Hanna Reitz , Vladimir Sementsov-Ogievskiy , Andrey Drobyshev Subject: Re: [PATCH 00/13] block: fix data loss in the preallocate filter Message-ID: References: <20260930165000.303236-1-den@openvz.org> <075b5093-afa5-4d4d-a8c5-8a1157fbeb62@virtuozzo.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <075b5093-afa5-4d4d-a8c5-8a1157fbeb62@virtuozzo.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 5 X-Spam_score: 0.5 X-Spam_bar: / X-Spam_report: (0.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Archive: List-Post: List-Help: List-Subscribe: , Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 Am 30.10.2026 um 12:12 hat Denis V. Lunev geschrieben: > On 9/30/26 18:49, Denis V. Lunev wrote: > > This email originated from an IP that might not be authorized by the domain it was sent from. > > Do not click links or open attachments unless it is an email you expected to receive. > > A guest running a container workload came up with 164 files of an image > > layer hollowed out, 45 MiB in all, reading back as zeroes where it had > > written data and been told the writes were on disk. The damage was > > scattered in guest offsets but contiguous in image file offsets: a > > qcow2 over something that zeroed a range of the file. > > > > The storage underneath records every change to the file's length. Over > > a tenth of a second the file grew thirteen times in unaligned steps, > > one per guest write past its end, then once by 82 MiB. A preallocation > > sets the length to the triggering write's end rounded up plus 128 MiB, > > so 82 cannot come from the length the file then had; it can only come > > from a zeroing that started where the burst began, 49 MiB lower, which > > is exactly the range that came back as zeroes. The fallocate ran over > > thirteen acknowledged writes. > > > > So the filter worked from a length the file no longer had. There are > > two ways to get there. Neither is proven, the record cannot decide > > between them, and both are fixed here. > > > > The first is that the filter meant to let those writes in. It raises > > data_end before issuing the preallocation, so that writes below it > > pass straight through into the range the fallocate is about to zero, > > trusting the block layer to order them after it. That trust is > > misplaced, and it needs no race to go wrong: two things meet. The > > file-posix widening that patch 1 removes parks the preallocation > > despite BDRV_REQ_NO_WAIT, and bdrv_find_conflicting_request() skips > > any overlapping request whose waiting_for is set without looking at > > what it points at, so a parked serialising request excludes nothing. > > > > The second is that the length was simply stale. The filter's lengths > > are plain fields, read, decided on and written back with nothing in > > between, and the guest that broke was a virtio-blk with its queues on > > two iothreads. A lost update on the reserved length gives a start 49 > > MiB low with no ordering subtlety at all. > > > > For the first, the straight fix is in bdrv_find_conflicting_request(), > > and it regresses iotests/077, whose "Overlapping multiple requests" > > case then verifies wrong data. Patch 1 takes away the other half > > instead: the XFS bug the workaround guards was fixed by 249bd9087a52 > > in v5.5, three days before the workaround went in, and predates every > > stable series still maintained. Without the widening NO_WAIT works as > > designed, so it is the one patch here I would not drop. > > > > For the second, the same state turns out to lose data three ways: > > > > - a write into the range the filter records as zero leaves the mark > > behind it, so a later write zeroes over that range is skipped and > > the guest reads the old data back; > > > > - two writers extending the file both pass the compare on data_end and > > the smaller end lands last, so closing the image truncates the file > > below acknowledged data; > > > > - two writers both preallocate over the same range, the older > > fallocate lands last, and the first writer's data is zeroed. Fixing > > this also stops the filter leaning on the ordering above. > > > > Two of the three have tests in iotests/298. The data_end race has > > none: there is no hook between the compare and the store to hang one > > on. > > > > Six patches are tooling, because none of this could be tested: every > > qemu-io request ran in the main loop, blkdebug could neither wake nor > > wait for a request on another AioContext, and nothing could hold a > > request before the block layer tracks it, which is the window the > > filter opens. None changes the output of an existing command. > > > > Signed-off-by: Denis V. Lunev > > CC: Kevin Wolf > > CC: Hanna Reitz > > CC: Vladimir Sementsov-Ogievskiy > > CC: Andrey Drobyshev > > > > Andrey Drobyshev (1): > > iotests/298: add testcase for async writes with preallocation filter > > > > Denis V. Lunev (12): > > block/file-posix: drop the XFS workaround for post-EOF fallocate > > block/preallocate: keep zero_start above written data > > block/preallocate: collect the state assignments in one place > > block/preallocate: serialise the filter state > > block/preallocate: hold back writes into a preallocation in flight > > tests/unit: prefix the iothread test helper > > qemu-io: accept --object iothread > > qemu-io: add -t to run an aio request in a named iothread > > qemu-io: add -g to write past the end of the image > > block/blkdebug: make a breakpoint work across AioContexts > > block: add blkdebug events for a write before it is tracked > > iotests/298: cover the preallocation races > > > > block/blkdebug.c | 3 +- > > block/file-posix.c | 34 -------- > > block/io.c | 3 + > > block/preallocate.c | 139 ++++++++++++++++++++++++------ > > meson.build | 2 +- > > qapi/block-core.json | 10 ++- > > qemu-io-cmds.c | 116 ++++++++++++++++++++++--- > > qemu-io.c | 41 ++++++++- > > tests/qemu-iotests/298 | 113 ++++++++++++++++++++++++ > > tests/qemu-iotests/298.out | 4 +- > > tests/unit/iothread.c | 28 +++--- > > tests/unit/iothread.h | 8 +- > > tests/unit/test-aio-multithread.c | 8 +- > > tests/unit/test-bdrv-drain.c | 32 +++---- > > tests/unit/test-block-iothread.c | 28 +++--- > > 15 files changed, 435 insertions(+), 134 deletions(-) > > > ping I went through the patches, but please don't forget Andrey's comments which he had already made before this ping. Kevin