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Thu, 6 Feb 2020 15:42:02 +0000 (UTC) Date: Thu, 6 Feb 2020 16:42:01 +0100 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH for-5.0 v2 10/23] quorum: Implement .bdrv_recurse_can_replace() Message-ID: <20200206154201.GF4926@linux.fritz.box> 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> MIME-Version: 1.0 In-Reply-To: <1bb2e344-e66d-de37-0d49-f4a8a5a6eb40@redhat.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="yLVHuoLXiP9kZBkt" Content-Disposition: inline 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: 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" --yLVHuoLXiP9kZBkt Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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.) > >=20 > > 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). > >=20 > >>> 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. > >=20 > > I'm not saying you need to do it, just that it would be cleaner. :-) >=20 > It would. Thanks for the suggestion, I obviously didn=E2=80=99t think of= it. > (Or there=E2=80=99d be a comment on how this is not the best way in theor= y, but > in practice it=E2=80=99s good enough.) I suppose I=E2=80=99ll see how wh= at I can do. >=20 > >>> 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. > >=20 > > Permissions are tied to file locks, so an external process can just gra= b > > the locks in between. >=20 > Ah, right, I didn=E2=80=99t think of that. >=20 > > 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. >=20 > Yes. I just think it actually would be 100 % in practice, so I wondered > whether it would need to be documented. >=20 > You=E2=80=99re right, though, it isn=E2=80=99t 100 %, so it should defini= tely be > documented. Maybe something like >=20 > 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. Sounds good until here. > 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. 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. 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