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Thu, 26 Sep 2019 11:17:38 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.40.205.151]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7E90160126; Thu, 26 Sep 2019 11:17:37 +0000 (UTC) Subject: Re: [PATCH 10/22] quorum: Implement .bdrv_recurse_can_replace() To: Vladimir Sementsov-Ogievskiy , "qemu-block@nongnu.org" References: <20190920152804.12875-1-mreitz@redhat.com> <20190920152804.12875-11-mreitz@redhat.com> <78926514-59db-f108-1ace-356a0bca8097@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: <702eee8e-80d9-2f6b-2a92-435cdcb48b68@redhat.com> Date: Thu, 26 Sep 2019 13:17:35 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.0 MIME-Version: 1.0 In-Reply-To: <78926514-59db-f108-1ace-356a0bca8097@virtuozzo.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="GhqZWih3nIkrfekjaaIJ93rBua1PXrB7D" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Thu, 26 Sep 2019 11:17:38 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 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 , Alberto Garcia , "qemu-devel@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) --GhqZWih3nIkrfekjaaIJ93rBua1PXrB7D Content-Type: multipart/mixed; boundary="G7EvafvFXtAaf7z8Q7VeJuFUAnFSgibSV" --G7EvafvFXtAaf7z8Q7VeJuFUAnFSgibSV Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 25.09.19 16:12, Vladimir Sementsov-Ogievskiy wrote: > 20.09.2019 18:27, Max Reitz wrote: >> Signed-off-by: Max Reitz >> --- >> block/quorum.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++= ++ >> 1 file changed, 62 insertions(+) >> >> diff --git a/block/quorum.c b/block/quorum.c >> index 207054a64e..81b57dbae2 100644 >> --- a/block/quorum.c >> +++ b/block/quorum.c >> @@ -825,6 +825,67 @@ static bool quorum_recurse_is_first_non_filter(Bl= ockDriverState *bs, >> return false; >> } >> =20 >> +static bool quorum_recurse_can_replace(BlockDriverState *bs, >> + BlockDriverState *to_replace) >> +{ >> + BDRVQuorumState *s =3D bs->opaque; >> + int i; >> + >> + for (i =3D 0; i < s->num_children; i++) { >> + /* >> + * We have no idea whether our children show the same data as= >> + * this node (@bs). It is actually highly likely that >> + * @to_replace does not, because replacing a broken child is >> + * one of the main use cases here. >> + * >> + * We do know that the new BDS will match @bs, so replacing >> + * any of our children by it will be safe. It cannot change >> + * the data this quorum node presents to its parents. >> + * >> + * However, replacing @to_replace by @bs in any of our >> + * children's chains may change visible data somewhere in >> + * there. We therefore cannot recurse down those chains with= >> + * bdrv_recurse_can_replace(). >> + * (More formally, bdrv_recurse_can_replace() requires that >> + * @to_replace will be replaced by something matching the @bs= >> + * passed to it. We cannot guarantee that.) >> + * >> + * Thus, we can only check whether any of our immediate >> + * children matches @to_replace. >> + * >> + * (In the future, we might add a function to recurse down a >> + * chain that checks that nothing there cares about a change >> + * in data from the respective child in question. For >> + * example, most filters do not care when their child's data >> + * suddenly changes, as long as their parents do not care.) >> + */ >> + if (s->children[i].child->bs =3D=3D to_replace) { >> + Error *local_err =3D NULL; >> + >> + /* >> + * We now have to ensure that there is no other parent >> + * that cares about replacing this child by a node with >> + * potentially different data. >> + */ >> + s->children[i].to_be_replaced =3D true; >> + bdrv_child_refresh_perms(bs, s->children[i].child, &local= _err); >> + >=20 > So we are trying to answer on a question "is it ok to replace" it, by c= heating on > permission system... Possibly, it's a problem of general design, and in= stead of > examining one subtree, we should ask all parents of to_replace node, = are they > OK with such replacement.. I=E2=80=99m not sure whether it=E2=80=99s cheating. We want to replace some node. A parent should be A-OK with that as long as it hasn=E2=80=99t frozen its child link, and as long as it doesn=E2=80= =99t care about data changes (it should not have taken CONSISTENT_READ, and it must have shared WRITE). The only actual problem we have is that currently basically everything takes CONSISTENT_READ (which is completely fine), but the only thing that doesn=E2=80=99t is the mirror_top_bs, and that has exactly the probl= em of =E2=80=9CI can only get away without CONSISTENT_READ if it was me who uns= hared it=E2=80=9D. But that=E2=80=99s a different problem. I don=E2=80=99t think this is ch= eating. > Another idea is that it's strange to check permissions somewhere else t= han in generic > permission check functions. But I've no idea how to handle it in permis= sion system. I don=E2=80=99t check the permissions, though. I let quorum take what it= needs to allow changing one of its children. What is a problem is that I should keep the permissions tightened until the node is actually replaced and only then release them. But that turned out to be a huge mess so I resorted to just double-checking before mirror actually completes. Max --G7EvafvFXtAaf7z8Q7VeJuFUAnFSgibSV-- --GhqZWih3nIkrfekjaaIJ93rBua1PXrB7D Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl2Mnk8ACgkQ9AfbAGHV z0Bn0gf9ExMMmK4ism69UmNmZcj2VDtGGSId0oumZ0hi0GhSzhswZdOzGQmT27UB 3/3dWngLw2Aeo2Celz/kUw+C3lhZ1qKOrHw8WXCsXpDMj4jqLs0O4z3gKA1DvDuM oY1mtDLGgadyiEzU901pu0/WU6BiV8QMll39Y39SFqorOGO7X7MSr/W+SDxfyAu2 Lq36C/9tuBzQTBU2ex+FY8LuL7Kg0SAxZ2AAIn0w4PWGoYu82qPzbKMqve/DHbyJ SGlgWShHD8GIQYI2uYaA7Y/3IhOMd5UHdsr1/Ub2I5+2vOyGokWb3gxE5jpzo8jP x9HN3oMShU0npfpZMYow96X7uZGuEQ== =hbJh -----END PGP SIGNATURE----- --GhqZWih3nIkrfekjaaIJ93rBua1PXrB7D--