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Thu, 20 Feb 2020 15:19:58 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.36.118.38]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DD1419051B; Thu, 20 Feb 2020 15:19:36 +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: <3d87fc6f-7f94-2da6-654a-a2dece6e5b5c@redhat.com> Date: Thu, 20 Feb 2020 16:19:33 +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="hn8gWREllaGZ18ZSddBsmPlsHZ7WKeDhT" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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) --hn8gWREllaGZ18ZSddBsmPlsHZ7WKeDhT Content-Type: multipart/mixed; boundary="HblUGXCbIhCxhdg8aTE1O2J7zDg2AtRBE" --HblUGXCbIhCxhdg8aTE1O2J7zDg2AtRBE Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable I have no idea how I managed to forgot some style comments I wrote during the review, but, anyway: 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 s/importante/important/ > 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(-) >=20 > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c > index e078bddcc2..9387f15866 100644 > --- a/block/qcow2-cluster.c > +++ b/block/qcow2-cluster.c [...] > @@ -1035,15 +1040,53 @@ void qcow2_alloc_cluster_abort(BlockDriverState *= bs, QCowL2Meta *m) > static void calculate_l2_meta(BlockDriverState *bs, > uint64_t host_cluster_offset, > uint64_t guest_offset, unsigned bytes, > - QCowL2Meta **m, bool keep_old) > + uint64_t *l2_slice, QCowL2Meta **m, bool k= eep_old) > { [...] > + /* Return if there's no COW (all clusters are normal and we keep the= m) */ > + if (keep_old) { > + int i; > + for (i =3D 0; i < nb_clusters; i++) { > + l2_entry =3D be64_to_cpu(l2_slice[l2_index + i]); > + if (qcow2_get_cluster_type(bs, l2_entry) !=3D QCOW2_CLUSTER_= NORMAL) { > + break; > + } > + } > + if (i =3D=3D nb_clusters) { > + return; > + } > + } > + > + /* Get the L2 entry from the first cluster */ s/from/of/ (Otherwise it sounds a bit like this is the same entry for all clusters) > + l2_entry =3D be64_to_cpu(l2_slice[l2_index]); > + type =3D qcow2_get_cluster_type(bs, l2_entry); > + > + if (type =3D=3D QCOW2_CLUSTER_NORMAL && keep_old) { > + cow_start_from =3D cow_start_to; > + } else { > + cow_start_from =3D 0; > + } > + > + /* Get the L2 entry from the last cluster */ s/from/of/ > + l2_entry =3D be64_to_cpu(l2_slice[l2_index + nb_clusters - 1]); > + type =3D qcow2_get_cluster_type(bs, l2_entry); > + > + if (type =3D=3D QCOW2_CLUSTER_NORMAL && keep_old) { > + cow_end_to =3D cow_end_from; > + } else { > + cow_end_to =3D ROUND_UP(cow_end_from, s->cluster_size); > + } > =20 > *m =3D g_malloc0(sizeof(**m)); > **m =3D (QCowL2Meta) { > @@ -1069,18 +1112,20 @@ static void calculate_l2_meta(BlockDriverState *b= s, > QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight); > } > =20 > -/* Returns true if writing to a cluster requires COW */ > -static bool cluster_needs_cow(BlockDriverState *bs, uint64_t l2_entry) > +/* Returns true if writing to the cluster pointed to by @l2_entry > + * requires a new allocation (that is, if the cluster is unallocated > + * or has refcount > 1 and therefore cannot be written in-place). */ Not sure why Patchew hasn=E2=80=99t complained, but the current coding styl= e requires /* and */ to be on separate lines for multi-line comments. 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