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Thu, 21 Nov 2019 12:21:47 +0000 (UTC) Subject: Re: [PATCH v2 2/6] block: truncate: Don't make backing file data visible To: Kevin Wolf References: <20191120184501.28159-1-kwolf@redhat.com> <20191120184501.28159-3-kwolf@redhat.com> <20191121113405.GE6007@linux.fritz.box> From: Max Reitz Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <38b48cd4-a7b6-c2c0-db38-99c2192b6d05@redhat.com> Date: Thu, 21 Nov 2019 13:21:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20191121113405.GE6007@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="KyGOvnj00T8OCttInSkgZCDsJ0Te92VJq" 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: vsementsov@virtuozzo.com, stefanha@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --KyGOvnj00T8OCttInSkgZCDsJ0Te92VJq Content-Type: multipart/mixed; boundary="P6ftp8sn6OZy5ijrRKv8969OURWBw2NKy" --P6ftp8sn6OZy5ijrRKv8969OURWBw2NKy Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 21.11.19 12:34, Kevin Wolf wrote: > Am 21.11.2019 um 09:59 hat Max Reitz geschrieben: >> On 20.11.19 19:44, 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. >>> >>> Consider the following scenario where the overlay is shorter than its >>> backing file: >>> >>> base.qcow2: AAAAAAAA >>> overlay.qcow2: BBBB >>> >>> When resizing (extending) overlay.qcow2, the new blocks should not stay >>> unallocated and make the additional As from base.qcow2 visible like >>> before this patch, but zeros should be read. >>> >>> A similar case happens with the various variants of a commit job when a= n >>> intermediate file is short (- for unallocated): >>> >>> base.qcow2: A-A-AAAA >>> mid.qcow2: BB-B >>> top.qcow2: C--C--C- >>> >>> After commit top.qcow2 to mid.qcow2, the following happens: >>> >>> mid.qcow2: CB-C00C0 (correct result) >>> mid.qcow2: CB-C--C- (before this fix) >>> >>> Without the fix, blocks that previously read as zeros on top.qcow2 >>> suddenly turn into A. >>> >>> Signed-off-by: Kevin Wolf >>> --- >>> block/io.c | 32 ++++++++++++++++++++++++++++++++ >>> 1 file changed, 32 insertions(+) >> >> Zeroing the intersection may take some time. So is it right for QMP=E2= =80=99s >> block_resize to do this, seeing it is a synchronous operation? >=20 > What else would be right? Returning an error? Going through a deprecation cycle. > Common cases (raw and qcow2 v3 without external data files) are quick > anyway. Well, but quick enough for a fully blocking operation? >> As far as I can tell, jobs actually have the same problem. I don=E2=80= =99t >> think mirror or commit have a pause point before truncating, so they >> still block the monitor there, don=E2=80=99t they? >=20 > Do you really need a pause point? They call bdrv_co_truncate() from > inside the job coroutine, so it will yield. I would expect that this > is enough. OK then. > But in fact, all jobs have a pause point before even calling .run(), so > even if that made a difference, it should still be fine. Good. But I believe this is still a problem for block_resize. I don=E2=80=99t se= e why this needs to be fixed in 4.2-rc3. What=E2=80=99s the problem with going through a proper deprecation cycle other than the fact that we can=E2=80=99= t start it in 4.2 because we don=E2=80=99t have a resize job yet? Max --P6ftp8sn6OZy5ijrRKv8969OURWBw2NKy-- --KyGOvnj00T8OCttInSkgZCDsJ0Te92VJq Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl3WgVoACgkQ9AfbAGHV z0DqQQgApPvFpDiqqgM5/l+QOPdEHvDS3bMBrR+LF8inDvD/Q4BlP1IkGDVIrTKY 5pZGx96djqNLcziccy71/dLcKk8OE2+rE9Fuu9TN+nOdaYFQdUGv60HrVngtDSIo PwGH2SlvpsKTkfrL6cEV/94qgN9f2gR7NHcQBvJrl0NMevMdp/UZpZSay8MHuEob 3G0Dm9510COGXAhiJyU3fmjXDv25N5J4WIEndw84GRVkQSlQL3CaSZbeKBLM9vfa idNVXWZCd+E1SIMQJzGDV/g97XzuGidDnM/iwJx0RP3SPuoIfsV1+bYsYwsbDSM4 5whtx6WiTXvfzSo+RG+dgnonAUcN1Q== =rDH/ -----END PGP SIGNATURE----- --KyGOvnj00T8OCttInSkgZCDsJ0Te92VJq--