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Thu, 20 Feb 2020 15:00:18 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.36.118.38]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 458DA8ED09; Thu, 20 Feb 2020 15:00:12 +0000 (UTC) Subject: Re: [RFC PATCH v3 03/27] qcow2: Process QCOW2_CLUSTER_ZERO_ALLOC clusters in handle_copied() To: Alberto Garcia , qemu-devel@nongnu.org References: 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: <3b7cdb8f-dd3e-879e-e4fc-0ecddc431a22@redhat.com> Date: Thu, 20 Feb 2020 16:00:01 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="3SbqjGOuzD4xppHbQgH1GLL7Uk6WYmPiL" X-detected-operating-system: by eggs.gnu.org: GNU/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: Kevin Wolf , Anton Nefedov , qemu-block@nongnu.org, Vladimir Sementsov-Ogievskiy , "Denis V . Lunev" 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) --3SbqjGOuzD4xppHbQgH1GLL7Uk6WYmPiL Content-Type: multipart/mixed; boundary="LMzoXjQSQEVA1yC7xnlBeQIO5PG81W6KZ" --LMzoXjQSQEVA1yC7xnlBeQIO5PG81W6KZ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 22.12.19 12:36, Alberto Garcia wrote: > When writing to a qcow2 file there are two functions that take a > virtual offset and return a host offset, possibly allocating new > clusters if necessary: >=20 > - handle_copied() looks for normal data clusters that are already > allocated and have a reference count of 1. In those clusters we > can simply write the data and there is no need to perform any > copy-on-write. >=20 > - handle_alloc() looks for clusters that do need copy-on-write, > either because they haven't been allocated yet, because their > reference count is !=3D 1 or because they are ZERO_ALLOC clusters. >=20 > The ZERO_ALLOC case is a bit special because those are clusters that > are already allocated and they could perfectly be dealt with in > handle_copied() (as long as copy-on-write is performed when required). >=20 > In fact, there is extra code specifically for them in handle_alloc() > that tries to reuse the existing allocation if possible and frees them > otherwise. >=20 > This patch changes the handling of ZERO_ALLOC clusters so the > semantics of these two functions are now like this: >=20 > - handle_copied() looks for clusters that are already allocated and > which we can overwrite (NORMAL and ZERO_ALLOC clusters with a > reference count of 1). >=20 > - handle_alloc() looks for clusters for which we need a new > allocation (all other cases). >=20 > One importante difference after this change is that clusters found in > handle_copied() may now require copy-on-write, but this will be anyway > necessary once we add support for subclusters. >=20 > Signed-off-by: Alberto Garcia > --- > block/qcow2-cluster.c | 226 +++++++++++++++++++++++------------------- > 1 file changed, 126 insertions(+), 100 deletions(-) Reviewed-by: Max Reitz --LMzoXjQSQEVA1yC7xnlBeQIO5PG81W6KZ-- --3SbqjGOuzD4xppHbQgH1GLL7Uk6WYmPiL Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl5OnvIACgkQ9AfbAGHV z0CVRQf/ZxX01zRk4Tf16rSCJ0YOWlWP7M/l1G4w4LpkuI7/iVon72qfLTo0GaUA 3joWdmLTvI2FDTVzdJYlu9YxplsZzzSk+CtydTRfqkVF29uLmUTQHzsu6Pd0a4yG 7TJAK3poY6I9GdmQmiqfPdnTVJjPnvOu3Cysjsuntv6x5efS7RjCsrWfDjoc/vp/ z8hmWfkNzQgaGPrlOX5nWzjYquYVxKT7wnxtvfKTLWTaLvLg9zsnEkuYkkDzlWEn MtWqja44k2WnpjboWSRjEWYp/lMecN+v3oSTBb4Qabp8qkGlMc8FtZkV5IUsRfVV yCSSTV0COuimq+GL0Yu9gHsJnKFUEA== =foSk -----END PGP SIGNATURE----- --3SbqjGOuzD4xppHbQgH1GLL7Uk6WYmPiL--