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Fri, 23 Aug 2019 17:53:31 +0000 (UTC) To: Nir Soffer References: <20190816212122.8816-1-nsoffer@redhat.com> <86c147ef-9dd3-d35a-5005-ec53081196d7@redhat.com> <271a2dd3-9f2b-013d-896f-f4059c804967@redhat.com> From: Max Reitz Openpgp: preference=signencrypt 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: <5a6cabf2-a284-9f63-da0e-96af26f8e0cd@redhat.com> Date: Fri, 23 Aug 2019 19:53:29 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="jPjcCvJ0BydNzknVNd4HI3ZB0zpIkcRlo" X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.67]); Fri, 23 Aug 2019 17:53:32 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH] block: posix: Always allocate the first block 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 , qemu-block , Nir Soffer , QEMU Developers 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) --jPjcCvJ0BydNzknVNd4HI3ZB0zpIkcRlo Content-Type: multipart/mixed; boundary="cJpxcVdjZW8Mq9qHOqC5aPZsSfVYrdmFJ"; protected-headers="v1" From: Max Reitz To: Nir Soffer Cc: Nir Soffer , qemu-block , Kevin Wolf , QEMU Developers Message-ID: <5a6cabf2-a284-9f63-da0e-96af26f8e0cd@redhat.com> Subject: Re: [PATCH] block: posix: Always allocate the first block References: <20190816212122.8816-1-nsoffer@redhat.com> <86c147ef-9dd3-d35a-5005-ec53081196d7@redhat.com> <271a2dd3-9f2b-013d-896f-f4059c804967@redhat.com> In-Reply-To: --cJpxcVdjZW8Mq9qHOqC5aPZsSfVYrdmFJ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 23.08.19 18:48, Nir Soffer wrote: > On Fri, Aug 23, 2019 at 4:58 PM Max Reitz > wrote: [...] > If you have a format layer that truncates the image to a fixed size= and > does not write anything into the first block itself (say because it= uses > a footer), then (with O_DIRECT) allocate_first_block() will fail > (silently, because while it does return an error value, it is never= > checked and there is no comment that explains why we don=E2=80=99t = check it) >=20 >=20 > The motivation is that this is an optimization for the special case of = using > empty image, so it does not worth failing image creation. > I will add a comment about that. Thanks! [...] > Thanks for the example. >=20 > I will need time to play with blockdev and understand the flows when im= age > are created. Do you think is would be useful to fix now only image crea= tion > via qemu-img, and handle blockdev later? Well, it isn=E2=80=99t about blockdev, it=E2=80=99s simply about the fact= that this function doesn=E2=80=99t work for O_DIRECT files. I showed how to reprod= uce the issue without blockdev (namely block_resize). Sure, that is an edge case, but it is a completely valid case. Also, it seems to me the fix is rather simple. Just something like: static int allocate_first_block(int fd, int64_t max_size) { int write_size =3D MIN(max_size, MAX_BLOCKSIZE); void *buf; ssize_t n; /* Round down to power of two */ assert(write_size > 0); write_size =3D 1 << (31 - clz32(write_size)); buf =3D qemu_memalign(MAX(getpagesize(), write_size), write_size); memset(buf, 0, write_size); do { n =3D pwrite(fd, buf, write_size, 0); } while (n < 0 && errno =3D=3D EINTR); qemu_vfree(buf); return n < 0 ? -errno : 0; } Wouldn=E2=80=99t that work? Max --cJpxcVdjZW8Mq9qHOqC5aPZsSfVYrdmFJ-- --jPjcCvJ0BydNzknVNd4HI3ZB0zpIkcRlo Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl1gKBkACgkQ9AfbAGHV z0Dtcwf/RxNkfuXZ2/43zlmj62xiTZRq/VQfQFzEUTFyCXbOGi6yqdAorLIUFQaR 8tEoV8WisGG0NNuZkr1y4jAGszKrZlwVBAV2HEt0OHv+CkCn9zYIfnhHkeJqvlSr 8iRvICovRbUS3vATmtxkbJm1A5o2Nw4gFp4+2/7PbcaxoZdzJoXWMMPWrLoxZurn BOvbzDM1l4vB2PkY+YrhntxbA3EWlD0MsrXS/3x2wuTUXmV+M/IjbP3YtQSnfOjF 1RuerbaAZjEFt0BTK78QL0yy2FPZvO3XpgfEu/rfvIK3K7z+IUG2rscPbI/3mhiN VC3RQXUKNB83xCjbC+yA281oNC7ejw== =tcWV -----END PGP SIGNATURE----- --jPjcCvJ0BydNzknVNd4HI3ZB0zpIkcRlo--