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Jones" To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH 4/4] block: introduce BDRV_MAX_LENGTH Message-ID: <20210107122005.GC2673@redhat.com> References: <20201203222713.13507-1-vsementsov@virtuozzo.com> <20201203222713.13507-5-vsementsov@virtuozzo.com> <20210107095817.GA2673@redhat.com> <20210107105611.GB2673@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210107105611.GB2673@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=rjones@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=63.128.21.124; envelope-from=rjones@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.246, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, 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: fam@euphon.net, kwolf@redhat.com, berto@igalia.com, qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, berrange@redhat.com, stefanha@redhat.com, den@openvz.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jan 07, 2021 at 10:56:12AM +0000, Richard W.M. Jones wrote: > On Thu, Jan 07, 2021 at 09:58:17AM +0000, Richard W.M. Jones wrote: > > On Fri, Dec 04, 2020 at 01:27:13AM +0300, Vladimir Sementsov-Ogievskiy wrote: > > > Finally to be safe with calculations, to not calculate different > > > maximums for different nodes (depending on cluster size and > > > request_alignment), let's simply set QEMU_ALIGN_DOWN(INT64_MAX, 2^30) > > > as absolute maximum bytes length for Qemu. Actually, it's not much less > > > than INT64_MAX. > > > > > +/* > > > + * We want allow aligning requests and disk length up to any 32bit alignment > > > + * and don't afraid of overflow. > > > + * To achieve it, and in the same time use some pretty number as maximum disk > > > + * size, let's define maximum "length" (a limit for any offset/bytes request and > > > + * for disk size) to be the greatest power of 2 less than INT64_MAX. > > > + */ > > > +#define BDRV_MAX_ALIGNMENT (1L << 30) > > > +#define BDRV_MAX_LENGTH (QEMU_ALIGN_DOWN(INT64_MAX, BDRV_MAX_ALIGNMENT)) > > > > This change broke nbdkit tests. > > Actually that's not the only problem. It appears that we're unable to > read or write the last sector of this disk: > > $ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -r -f raw "$uri" -c "r -v $(( 2**63 - 2**30 - 512 )) 512" ' > read failed: Input/output error > > $ nbdkit memory $(( 2**63 - 2**30 )) --run 'build/qemu-io -f raw "$uri" -c "w -P 3 $(( 2**63 - 2**30 - 512 )) 512" ' > write failed: Input/output error > > You can play around with the constants. I found it's possible to read > and write the non-aligned 512 bytes starting at 2^63-2^30-513. Could > be a fencepost error somewhere in qemu? Actually this is a pre-existing bug in qemu. What happens is qemu-io calls qemu_strtosz("9223372035781033472") which returns 0x7fffffffc0000000 and no error. That answer is plain flat out wrong. The reason for that is qemu_strtosz uses floating point for the calculation(!) so is limited to 53 bits of precision and silently truncates. It happened we were just lucky that our earlier test with 2^63 - 1024 worked. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org