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Thu, 6 Feb 2020 16:44:56 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.36.118.15]) by smtp.corp.redhat.com (Postfix) with ESMTPS id DE5AE60BEC; Thu, 6 Feb 2020 16:44:53 +0000 (UTC) Subject: Re: [PATCH for-5.0 v2 10/23] quorum: Implement .bdrv_recurse_can_replace() To: Kevin Wolf References: <20191111160216.197086-1-mreitz@redhat.com> <20191111160216.197086-11-mreitz@redhat.com> <20200205155511.GF5768@dhcp-200-226.str.redhat.com> <7429d107-63c0-b6e4-5047-d17e9d510efc@redhat.com> <20200206144207.GC4926@linux.fritz.box> <1bb2e344-e66d-de37-0d49-f4a8a5a6eb40@redhat.com> <20200206154201.GF4926@linux.fritz.box> 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: Thu, 6 Feb 2020 17:44:52 +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: <20200206154201.GF4926@linux.fritz.box> 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="8q1orth5i2LfKUowuKBKQULOvGCIdDqas" 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: Vladimir Sementsov-Ogievskiy , Alberto Garcia , qemu-devel@nongnu.org, 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) --8q1orth5i2LfKUowuKBKQULOvGCIdDqas Content-Type: multipart/mixed; boundary="M8WLGp7wvar6b9gsq9lDqBKSu049slQ2H" --M8WLGp7wvar6b9gsq9lDqBKSu049slQ2H Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 06.02.20 16:42, Kevin Wolf wrote: > Am 06.02.2020 um 16:19 hat Max Reitz geschrieben: >> On 06.02.20 15:42, Kevin Wolf wrote: >>> Am 06.02.2020 um 11:21 hat Max Reitz geschrieben: >>>> On 05.02.20 16:55, Kevin Wolf wrote: >>>>> Am 11.11.2019 um 17:02 hat Max Reitz geschrieben: >>>>>> 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 3a824e77e3..8ee03e9baf 100644 >>>>>> --- a/block/quorum.c >>>>>> +++ b/block/quorum.c >>>>>> @@ -825,6 +825,67 @@ static bool quorum_recurse_is_first_non_filter(= BlockDriverState *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 i= s >>>>>> + * 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 chang= e >>>>>> + * 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 wi= th >>>>>> + * 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 chang= e >>>>>> + * in data from the respective child in question. For >>>>>> + * example, most filters do not care when their child's dat= a >>>>>> + * 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, &loc= al_err); >>>>>> + >>>>>> + /* Revert permissions */ >>>>>> + s->children[i].to_be_replaced =3D false; >>>>>> + bdrv_child_refresh_perms(bs, s->children[i].child, &err= or_abort); >>>>> >>>>> Quite a hack. The two obvious problems are: >>>>> >>>>> 1. We can't guarantee that we can actually revert the permissions. I >>>>> think we ignore failure to loosen permissions meanwhile so that at >>>>> least the &error_abort doesn't trigger, but bs could still be in t= he >>>>> wrong state afterwards. >>>> >>>> I thought we guaranteed that loosening permissions never fails. >>>> >>>> (Well, you know. It may =E2=80=9Cleak=E2=80=9D permissions, but we=E2= =80=99d never get an error >>>> here so there=E2=80=99s nothing to handle anyway.) >>> >>> This is what I meant. We ignore the failure (i.e. don't return an error= ), >>> but the result still isn't completely correct ("leaked" permissions). >>> >>>>> It would be cleaner to use check+abort instead of actually setting >>>>> the new permission. >>>> >>>> Oh. Yes. Maybe. It does require more code, though, because I=E2=80= =99d rather >>>> not use bdrv_check_update_perm() from here as-is. >>> >>> I'm not saying you need to do it, just that it would be cleaner. :-) >> >> It would. Thanks for the suggestion, I obviously didn=E2=80=99t think o= f it. >> (Or there=E2=80=99d be a comment on how this is not the best way in theo= ry, but >> in practice it=E2=80=99s good enough.) I suppose I=E2=80=99ll see how w= hat I can do. >> >>>>> 2. As aborting the permission change makes more obvious, we're checki= ng >>>>> something that might not be true any more when we actually make th= e >>>>> change. >>>> >>>> True. I tried to do it right by having a post-replace cleanup functio= n, >>>> but after a while that was just going nowhere, really. So I just went >>>> with what=E2=80=99s patch 13 here. >>>> >>>> But isn=E2=80=99t 13 enough, actually? It check can_replace right bef= ore >>>> replacing in a drained section. I can=E2=80=99t imagine the permissio= ns to >>>> change there. >>> >>> Permissions are tied to file locks, so an external process can just gra= b >>> the locks in between. >> >> Ah, right, I didn=E2=80=99t think of that. >> >>> But if I understand correctly, all we try here is >>> to have an additional safeguard to prevent the user from doing stupid >>> things. So I guess not being 100% is fine as long as it's documented in >>> the code. >> >> Yes. I just think it actually would be 100 % in practice, so I wondered >> whether it would need to be documented. >> >> You=E2=80=99re right, though, it isn=E2=80=99t 100 %, so it should defin= itely be >> documented. Maybe something like >> >> In theory, we would have to keep the permissions tightened until the >> node is replaced. In practice, that would require post-replacement >> cleanup infrastructure, which we do not have, and which would be >> unreasonably complex to implement. >=20 > Sounds good until here. >=20 >> Therefore, all we can do is require >> anyone who wants to replace one node by some potentially unrelated other >> node (i.e., the mirror job on completion) to invoke >> bdrv_recurse_can_replace() immediately before and thus minimize the time >> during which some condition may arise that might forbid the swap. >> >> ? >=20 > This second part of your suggested comment could be dropped, as far as > I'm concerned. If anything, it's part of the contract and would belong > in the bdrv_recurse_can_replace() documentation. >=20 > However, I think I would mention why not being 100% is okay: The part > with "additional safeguard to prevent the user from doing stupid > things", and that it doesn't make a difference if the user runs the > correct command. OK. 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