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Thu, 23 Apr 2020 13:23:06 +0000 (UTC) Received: from linux.fritz.box (ovpn-114-28.ams2.redhat.com [10.36.114.28]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 26D785C1BE; Thu, 23 Apr 2020 13:23:03 +0000 (UTC) Date: Thu, 23 Apr 2020 15:23:02 +0200 From: Kevin Wolf To: Eric Blake Subject: Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate Message-ID: <20200423132302.GA23654@linux.fritz.box> References: <20200422152129.167074-1-kwolf@redhat.com> <20200422152129.167074-5-kwolf@redhat.com> <84c6ca49-aef3-adf4-2efe-68357dd20ce8@redhat.com> <20200422155835.GC7155@linux.fritz.box> <20d12788-2d62-12a4-0949-f29594054026@redhat.com> MIME-Version: 1.0 In-Reply-To: <20d12788-2d62-12a4-0949-f29594054026@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable 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 = 2020/04/23 03:23:21 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 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: vsementsov@virtuozzo.com, berto@igalia.com, qemu-devel@nongnu.org, qemu-block@nongnu.org, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 22.04.2020 um 18:14 hat Eric Blake geschrieben: > On 4/22/20 10:58 AM, Kevin Wolf wrote: >=20 > > > > @@ -4214,6 +4215,35 @@ static int coroutine_fn qcow2_co_truncate(Bl= ockDriverState *bs, int64_t offset, > > > > g_assert_not_reached(); > > > > } > > > > + if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) { > > > > + uint64_t zero_start =3D QEMU_ALIGN_UP(old_length, s->clust= er_size); > > > > + uint64_t zero_end =3D QEMU_ALIGN_UP(offset, s->cluster_siz= e); > > >=20 > > > This rounds up beyond the new size... > > >=20 > > > > + > > > > + /* Use zero clusters as much as we can */ > > > > + ret =3D qcow2_cluster_zeroize(bs, zero_start, zero_end - z= ero_start, 0); > > >=20 > > > and then requests that the extra be zeroed. Does that always work, e= ven > > > when it results in pdrv_co_pwrite_zeroes beyond the end of s->data_fi= le? > >=20 > > You mean the data_file_is_raw() path in qcow2_cluster_zeroize()? It's > > currently not a code path that is run because we only set > > BDRV_REQ_ZERO_WRITE for truncate if the image has a backing file, and > > data_file_is_raw() doesn't work with backing files. >=20 > Good point. >=20 > >=20 > > But hypothetically, if someone called truncate with BDRV_REQ_ZERO_WRITE > > for such a file, I think it would fail. > >=20 > > > If so, > > >=20 > > > Reviewed-by: Eric Blake > > >=20 > > > otherwise, you may have to treat the tail specially, the same way you > > > treated an unaligned head. > >=20 > > Actually, do I even need to round the tail? > >=20 > > /* Caller must pass aligned values, except at image end */ > > assert(QEMU_IS_ALIGNED(offset, s->cluster_size)); > > assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) || > > end_offset =3D=3D bs->total_sectors << BDRV_SECTOR_BITS); > >=20 > > So qcow2_cluster_zeroize() seems to accept the unaligned tail. It would > > still set the zero flag for the partial last cluster and for the > > external data file, bdrv_co_pwrite_zeroes() would have the correct size= . >=20 > Then I'm in favor of NOT rounding the tail. That's an easy enough change > and we've now justified that it does what we want, so R-b stands with tha= t > one-line tweak. Would have been too easy... bs->total_sectors isn't updated yet, so the assertion does fail. I can make the assertion check end_offset >=3D ... instead. That should still check what we wanted to check here and allow the unaligned extension. This feels like the better option to me compared to updating bs->total_sectors earlier and then undoing that change in every error path. Kevin