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Wed, 5 Feb 2020 17:26:02 +0000 (UTC) Subject: Re: [PATCH 10/17] block: Add new BDRV_ZERO_OPEN flag To: Eric Blake , qemu-devel@nongnu.org References: <20200131174436.2961874-1-eblake@redhat.com> <20200131174436.2961874-11-eblake@redhat.com> <5c19c0fe-f8d0-5011-7cc6-4bb46a46cedf@redhat.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: Date: Wed, 5 Feb 2020 18:26:00 +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: <5c19c0fe-f8d0-5011-7cc6-4bb46a46cedf@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="S3hRi2cz3HpiucKEEaNXqe1y2yHy1i0KZ" 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: david.edmondson@oracle.com, Kevin Wolf , qemu-block@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) --S3hRi2cz3HpiucKEEaNXqe1y2yHy1i0KZ Content-Type: multipart/mixed; boundary="PjBg2Jdjo2Fd3gRvXxxpjulj5h7XmJ3rO" --PjBg2Jdjo2Fd3gRvXxxpjulj5h7XmJ3rO Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 04.02.20 18:50, Eric Blake wrote: > On 2/4/20 11:34 AM, Max Reitz wrote: >=20 >>> +++ b/include/block/block.h >>> @@ -105,6 +105,16 @@ typedef enum { >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * for drivers that set .bdrv_co_tr= uncate. >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */ >>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 BDRV_ZERO_TRUNCATE=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0 =3D 0x2, >>> + >>> +=C2=A0=C2=A0=C2=A0 /* >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * bdrv_known_zeroes() should include this bit= if an image is >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * known to read as all zeroes when first open= ed; this bit should >>> +=C2=A0=C2=A0=C2=A0=C2=A0 * not be relied on after any writes to the im= age. >> >> Is there a good reason for this?=C2=A0 Because to me this screams like w= e are >> going to check this flag without ensuring that the image has actually >> not been written to yet.=C2=A0 So if it=E2=80=99s generally easy for dri= vers to stop >> reporting this flag after a write, then maybe we should do so. >=20 > In patch 15 (implementing things in qcow2), I actually wrote the driver > to return live results, rather than just open-time results, in part > because writing the bit to persistent storage in qcow2 means that the > bit must be accurate, without relying on the block layer's help. >=20 > But my pending NBD patch (not posted yet, but will be soon), the > proposal I'm making for the NBD protocol itself is just open-time, not > live, and so it would be more work than necessary to make the NBD driver > report live results. >=20 > But it seems like it should be easy enough to also patch the block layer > itself to guarantee that callers of bdrv_known_zeroes() cannot see this > bit set if the block layer has been used in any non-zero transaction, by > repeating the same logic as used in qcow2 to kill the bit (any > write/write_compressed/bdrv_copy clear the bit, any trim clears the bit > if the driver does not guarantee trim reads as zero, any truncate clears > the bit if the driver does not guarantee truncate reads as zero, etc). > Basically, the block layer would cache the results of .bdrv_known_zeroes > during .bdrv_co_open, bdrv_co_pwrite() and friends would update that > cache, and and bdrv_known_zeroes() would report the cached value rather > than a fresh call to .bdrv_known_zeroes. Sounds reasonable to me in generaly, but I=E2=80=99d prefer it to be fetche= d on-demand rather than unconditionally in bdrv_open(). (I realize that this means a tri-state of =E2=80=9Cknown false=E2=80=9D, = =E2=80=9Cknown true=E2=80=9D, and =E2=80=9Cnot yet queried=E2=80=9D.) > Are we worried enough about clients of this interface to make the block > layer more robust?=C2=A0 (From the maintenance standpoint, the more the b= lock > layer guarantees, the easier it is to write code that uses the block > layer; but there is the counter-argument that making the block layer > track whether an image has been modified means a [slight] penalty to > every write request to update the boolean.) Just like Vladimir, I=E2=80=99m worried about repeating the same mistakes w= e have before: That is, most places that called bdrv_has_zero_init() just did so out of wishful thinking, hoping that it would do what they need it to. It didn=E2=80=99t. Max --PjBg2Jdjo2Fd3gRvXxxpjulj5h7XmJ3rO-- --S3hRi2cz3HpiucKEEaNXqe1y2yHy1i0KZ Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl46+qgACgkQ9AfbAGHV z0BAJgf8DWqwE9Ft51SdRC4ttZfHolMwqwLQ11CMIT2GaejyMWdf7yrdUdkWHLfs yTGBLVXHjvWjUHDouu3XFhbBx4C56AzVOT122PkA+8WUWRBZmhwVhaN+2kE7Sgfn nB2OIVsyDtpKk5GgberVxW3Jt8Fxw4jnr9RKXxZBeqqgKL+H3HxEzKk5nNBuyBz1 VTU868pqAbB4PS9SjElfYelKwkwyb8Z69eJRTan3MXkE0KJMOUyJiQkq4sEq/ifT Witp+ZCSG6YZEMyGMemQiw1suTzVX9Oo9gnvnLBFSchL7Gl2y5XpfM4Wj1waqJgw ADByaZKiDKX3q7sMPj+iA+/3dub1Rg== =EOyf -----END PGP SIGNATURE----- --S3hRi2cz3HpiucKEEaNXqe1y2yHy1i0KZ--