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Wed, 20 Nov 2019 15:12:36 +0000 (UTC) Date: Wed, 20 Nov 2019 16:12:35 +0100 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH 2/6] block: truncate: Don't make backing file data visible Message-ID: <20191120151235.GD5779@linux.fritz.box> References: <20191120140319.1505-1-kwolf@redhat.com> <20191120140319.1505-3-kwolf@redhat.com> <5f37eeef-2a2c-dcdc-0e2c-b1d49bc9a3d7@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <5f37eeef-2a2c-dcdc-0e2c-b1d49bc9a3d7@virtuozzo.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: kfdm0NizPW-FAD7n757ORA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] 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: "qemu-devel@nongnu.org" , "stefanha@redhat.com" , "qemu-block@nongnu.org" , "mreitz@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 20.11.2019 um 15:47 hat Vladimir Sementsov-Ogievskiy geschrieben: > 20.11.2019 17:03, Kevin Wolf wrote: > > When extending the size of an image that has a backing file larger than > > its old size, make sure that the backing file data doesn't become > > visible in the guest, but the added area is properly zeroed out. > >=20 > > The old behaviour made a difference in 'block_resize' (where showing th= e > > backing file data from an old snapshot rather than zeros is > > questionable) as well as in commit block jobs (both from active and > > intermediate nodes) and HMP 'commit', where committing to a short > > backing file would incorrectly omit writing zeroes for unallocated > > blocks on the top layer after the EOF of the short backing file. > >=20 > > Signed-off-by: Kevin Wolf > > --- > > block/io.c | 25 +++++++++++++++++++++++++ > > 1 file changed, 25 insertions(+) > >=20 > > diff --git a/block/io.c b/block/io.c > > index 003f4ea38c..8683f7a4bd 100644 > > --- a/block/io.c > > +++ b/block/io.c > > @@ -3382,6 +3382,31 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *chi= ld, int64_t offset, bool exact, > > goto out; > > } > > =20 > > + /* > > + * If the image has a backing file that is large enough that it wo= uld > > + * provide data for the new area, we cannot leave it unallocated b= ecause > > + * then the backing file content would become visible. Instead, ze= ro-fill > > + * the area where backing file and new area overlap. > > + */ >=20 > Should we mention that, still, we don't care if user for some reason will= change > backing file in future? This should be obvious, but I can add something. > > + if (new_bytes && bs->backing && prealloc =3D=3D PREALLOC_MODE_OFF)= { > > + int64_t backing_len; > > + > > + backing_len =3D bdrv_getlength(backing_bs(bs)); > > + if (backing_len < 0) { > > + ret =3D backing_len; > > + goto out; > > + } > > + > > + if (backing_len > old_size) { > > + ret =3D bdrv_co_do_pwrite_zeroes(bs, old_size, > > + MIN(new_bytes, backing_len = - old_size), > > + BDRV_REQ_ZERO_WRITE | BDRV_= REQ_MAY_UNMAP); >=20 > two over-80 lines Will fix. > > + if (ret < 0) { > > + goto out; > > + } > > + } > > + } >=20 > should we improve "off" mode specification in qapi? I don't think we're changing the semantics of "off". We're merely fixing a bug that happens not to exist with preallocation. > > + > > ret =3D refresh_total_sectors(bs, offset >> BDRV_SECTOR_BITS); > > if (ret < 0) { > > error_setg_errno(errp, -ret, "Could not refresh total sector = count"); > >=20 >=20 > Hmm. is it correct to call write_zeroes before refresh_total_sectors? > Note that qcow2_co_pwrite_zeroes rely on bs->total_sectors... Hm... I placed the code where I did because I didn't want to make the new area valid before it isn't zeroed. But you're probably right that we shouldn't run requests with inconsistent bs->total_sectors, so I'll switch the order. Kevin