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Tue, 14 Jul 2020 11:08:38 +0000 (UTC) Date: Tue, 14 Jul 2020 13:08:37 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH for-5.1 1/2] block: Require aligned image size to avoid assertion failure Message-ID: <20200714110837.GB5120@linux.fritz.box> References: <20200710142149.40962-1-kwolf@redhat.com> <20200710142149.40962-2-kwolf@redhat.com> <20200713142949.GD10318@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="gKMricLos+KVdGMg" Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/14 03:57:32 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: nsoffer@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --gKMricLos+KVdGMg Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 14.07.2020 um 11:56 hat Max Reitz geschrieben: > On 13.07.20 16:29, Kevin Wolf wrote: > > Am 13.07.2020 um 13:19 hat Max Reitz geschrieben: > >> On 10.07.20 16:21, Kevin Wolf wrote: > >>> Unaligned requests will automatically be aligned to bl.request_alignm= ent > >>> and we don't want to extend requests to access space beyond the end o= f > >>> the image, so it's required that the image size is aligned. > >>> > >>> With write requests, this could cause assertion failures like this if > >>> RESIZE permissions weren't requested: > >>> > >>> qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion `end_= sector <=3D bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed. > >>> > >>> This was e.g. triggered by qemu-img converting to a target image with= 4k > >>> request alignment when the image was only aligned to 512 bytes, but n= ot > >>> to 4k. > >>> > >>> Signed-off-by: Kevin Wolf > >>> --- > >>> block.c | 10 ++++++++++ > >>> 1 file changed, 10 insertions(+) > >> > >> (I think we had some proposal like this before, but I can=E2=80=99t fi= nd it, > >> unfortunately...) > >> > >> I can=E2=80=99t see how with this patch you could create qcow2 images = and then > >> use them with direct I/O, because AFAICS, qemu-img create doesn=E2=80= =99t allow > >> specifying caching options, so AFAIU you=E2=80=99re stuck with: > >> > >> $ ./qemu-img create -f qcow2 /mnt/tmp/foo.qcow2 1M > >> Formatting '/mnt/tmp/foo.qcow2', fmt=3Dqcow2 cluster_size=3D65536 > >> compression_type=3Dzlib size=3D1048576 lazy_refcounts=3Doff refcount_b= its=3D16 > >> > >> $ sudo ./qemu-io -t none /mnt/tmp/foo.qcow2 > >> qemu-io: can't open device /mnt/tmp/foo.qcow2: Image size is not a > >> multiple of request alignment > >> > >> (/mnt/tmp is a filesystem on a =E2=80=9Closetup -b 4096=E2=80=9D devic= e.) > >=20 > > Hm, that looks like some regrettable collateral damage... > >=20 > > Well, you could argue that we should be writing full L1 tables with zer= o > > padding instead of just the used part. I thought we had fixed this long > > ago. But looks like we haven't. >=20 > That would help for the standard case. It wouldn=E2=80=99t when the clus= ter > size is smaller than the request alignment, which, while maybe not > important, would still be a shame. I don't think it would be unreasonable to require a cluster size that is a multiple of the logical block size of your host storage if you want to use O_DIRECT. But we have unaligned images in practice, so this is pure theory anyway. > > But we should still avoid crashing in other cases, so what is the > > difference between both? Is it just that qcow2 has the RESIZE permissio= n > > anyway so it doesn't matter? >=20 > I assume so. >=20 > > If so, maybe attaching to a block node with WRITE, but not RESIZE is > > what needs to fail when the image size is unaligned? >=20 > That sounds reasonable. >=20 > The obvious question is what happens when the RESIZE capability is > removed. Dropping capabilities may never fail =E2=80=93 I suppose we cou= ld > force-keep the RESIZE capability for such nodes? It's not nice, but I think we already have this kind of behaviour for unlocking failures. So yes, that sounds like an option. > Or we could immediately align such files to the block size once they > are opened (with the RESIZE capability). Automatically resizing the image file is obviously harmless for qcow2 images, but it would be a guest-visible change for raw images. It might be better to avoid this. 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