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Tue, 4 Feb 2020 17:42:40 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-117-98.ams2.redhat.com [10.36.117.98]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 4651760BF4; Tue, 4 Feb 2020 17:42:29 +0000 (UTC) Subject: Re: [PATCH 09/17] block: Refactor bdrv_has_zero_init{,_truncate} To: Vladimir Sementsov-Ogievskiy , Eric Blake , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> <20200131174436.2961874-10-eblake@redhat.com> <339f0a60-1e4f-286c-6594-1153bf284082@virtuozzo.com> 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: <62078ab7-b46f-cb70-ffb1-5e84c7e1bee5@redhat.com> Date: Tue, 4 Feb 2020 18:42:27 +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: <339f0a60-1e4f-286c-6594-1153bf284082@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="PFkSDciiVByunOTp7J81h18f4JboJlQnN" X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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 , Fam Zheng , "open list:Sheepdog" , qemu-block@nongnu.org, Jeff Cody , Stefan Weil , Peter Lieven , "Richard W.M. Jones" , Markus Armbruster , david.edmondson@oracle.com, Stefan Hajnoczi , "Denis V. Lunev" , Liu Yuan , Jason Dillaman 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) --PFkSDciiVByunOTp7J81h18f4JboJlQnN Content-Type: multipart/mixed; boundary="5wchGUj0lWjstJkMcvJC2jVzaEx4Wh5Yd" --5wchGUj0lWjstJkMcvJC2jVzaEx4Wh5Yd Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 04.02.20 16:35, Vladimir Sementsov-Ogievskiy wrote: > 31.01.2020 20:44, Eric Blake wrote: >> Having two slightly-different function names for related purposes is >> unwieldy, especially since I envision adding yet another notion of >> zero support in an upcoming patch.=C2=A0 It doesn't help that >> bdrv_has_zero_init() is a misleading name (I originally thought that a >> driver could only return 1 when opening an already-existing image >> known to be all zeroes; but in reality many drivers always return 1 >> because it only applies to a just-created image).=C2=A0 Refactor all use= s >> to instead have a single function that returns multiple bits of >> information, with better naming and documentation. >=20 > Sounds good >=20 >> >> No semantic change, although some of the changes (such as to qcow2.c) >> require a careful reading to see how it remains the same. >> >=20 > ... >=20 >> diff --git a/include/block/block.h b/include/block/block.h >> index 6cd566324d95..a6a227f50678 100644 >> --- a/include/block/block.h >> +++ b/include/block/block.h >=20 > Hmm, header file in the middle of the patch, possibly you don't use > [diff] > =C2=A0=C2=A0=C2=A0 orderFile =3D scripts/git.orderfile >=20 > in git config.. Or it is broken. >=20 >> @@ -85,6 +85,28 @@ typedef enum { >> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 BDRV_REQ_MASK=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 =3D 0x3ff, >> =C2=A0 } BdrvRequestFlags; >> >> +typedef enum { >> +=C2=A0=C2=A0=C2=A0 /* >> +=C2=A0=C2=A0=C2=A0=C2=A0 * bdrv_known_zeroes() should include this bit = if the contents of >> +=C2=A0=C2=A0=C2=A0=C2=A0 * a freshly-created image with no backing file= reads as all >> +=C2=A0=C2=A0=C2=A0=C2=A0 * zeroes without any additional effort.=C2=A0 = If .bdrv_co_truncate is >> +=C2=A0=C2=A0=C2=A0=C2=A0 * set, then this must be clear if BDRV_ZERO_TR= UNCATE is clear. >=20 > I understand that this is preexisting logic, but could I ask: why? > What's wrong > if driver can guarantee that created file is all-zero, but is not sure > about > file resizing? I agree that it's normal for these flags to have the same > value, > but what is the reason for this restriction?.. If areas added by truncation (or growth, rather) are always zero, then the file can always be created with size 0 and grown from there. Thus, images where truncation adds zeroed areas will generally always be zero after creation. > So, the only possible combination of flags, when they differs, is > create=3D0 and > truncate=3D1.. How is it possible? For preallocated qcow2 images, it depends on the storage whether they are actually 0 after creation. Hence qcow2_has_zero_init() then defers to bdrv_has_zero_init() of s->data_file->bs. But when you truncate them (with PREALLOC_MODE_OFF, as BlockDriver.bdrv_has_zero_init_truncate()=E2=80=99s comment explains), the = new area is always going to be 0, regardless of initial preallocation. I just noticed a bug there, though: Encrypted qcow2 images will not see areas added through growth as 0. Hence, qcow2=E2=80=99s bdrv_has_zero_init_truncate() implementation should not return true unconditionally, but only for unencrypted images. Max --5wchGUj0lWjstJkMcvJC2jVzaEx4Wh5Yd-- --PFkSDciiVByunOTp7J81h18f4JboJlQnN Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl45rQMACgkQ9AfbAGHV z0AhUgf7BXPMYMsDabZZLLiI4T7Nev5QGO7VPFq1dEk8toWTtQO/1KLPo7PQZyw4 qlaaYIsZONA+jnV8DNu+OYj7qy3SWQRajfZs4j+VPxLmsI3+9UZMAV7MzOKjWCg0 NgnqJ1NyxVP29V7/drJQ0UgCsnGUAUE99WGTGfe9i/MbKjf0VjIvWQlCKpmDhJvu Dmw+RR06b/g0ycT04klIBr1/UkTK7jacppFHxxvfT+YvS1pnyKxZzg4UcBd15GnV /c4VJHhouQsq/8gO5+pU8gjIXvmrZCm4mNG64XWMoCgGgIY1a0MtTEUhe56GtZbs E+qV5HNsVLTLExyDb0XXtrv2kyHjQQ== =yh34 -----END PGP SIGNATURE----- --PFkSDciiVByunOTp7J81h18f4JboJlQnN--