Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (address@hidden) wrote:
> On Mon, May 11, 2020 at 01:07:18PM +0100, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrangé (address@hidden) wrote:
> > > On Mon, May 11, 2020 at 01:14:34PM +0200, Lukas Straub wrote:
> > > > Hello Everyone,
> > > > In many cases, if qemu has a network connection (qmp, migration, 
> > > > chardev, etc.)
> > > > to some other server and that server dies or hangs, qemu hangs too.
> > > 
> > > If qemu as a whole hangs due to a stalled network connection, that is a
> > > bug in QEMU that we should be fixing IMHO. QEMU should be doing 
> > > non-blocking
> > > I/O in general, such that if the network connection or remote server 
> > > stalls,
> > > we simply stop sending I/O - we shouldn't ever hang the QEMU process or 
> > > main
> > > loop.
> > > 
> > > There are places in QEMU code which are not well behaved in this respect,
> > > but many are, and others are getting fixed where found to be important.
> > > 
> > > Arguably any place in QEMU code which can result in a hang of QEMU in the
> > > event of a stalled network should be considered a security flaw, because
> > > the network is untrusted in general.
> > 
> > That's not really true of the 'management network' - people trust that
> > and I don't see a lot of the qemu code getting fixed safely for all of
> > them.
> 
> It depends on the user / app / deployment scenario. In OpenStack alot of
> work was done to beef up security between services on the mgmt network,
> with TLS encryption as standard to reduce attack vectors.
> 
> > > > These patches introduce the new 'yank' out-of-band qmp command to 
> > > > recover from
> > > > these kinds of hangs. The different subsystems register callbacks which 
> > > > get
> > > > executed with the yank command. For example the callback can shutdown() 
> > > > a
> > > > socket. This is intended for the colo use-case, but it can be used for 
> > > > other
> > > > things too of course.
> > > 
> > > IIUC, invoking the "yank" command unconditionally kills every single
> > > network connection in QEMU that has registered with the "yank" subsystem.
> > > IMHO this is way too big of a hammer, even if we accept there are bugs in
> > > QEMU not handling stalled networking well.
> > 
> > But isn't this hammer conditional - I see that it's a migration
> > capabiltiy for the migration socket, and a flag in nbd - so it only
> > yanks things you've told it to.
> 
> IIUC, you have to set these flags upfront when you launch QEMU, or
> hotplug the device using the feature. When something gets stuck,
> and you issue the "yank" command, then everything that has the flag
> enabled gets torn down. So in practice it looks like the flag will
> get enabled for everything at QEMU startup, and yanking down tear
> down everything.

For COLO I really expect it for the migration stream, the disk mirroring
stream and probably the network comparison/forwarding streams.

> > > eg if a chardev hangs QEMU, and we tear down everything, killing the NBD
> > > connection used for the guest disk, we needlessly break I/O.
> > > 
> > > eg doing this in the chardev backend is not desirable, because the bugs
> > > with hanging QEMU are typically caused by the way the frontend device
> > > uses the chardev blocking I/O calls, instead of non-blocking I/O calls.
> > > 
> > 
> > Having a way to get out of any of these problems from a single point is
> > quite nice.  To be useful in COLO you need to know for sure you can get
> > out of any network screwup.
> > 
> > We already use shutdown(2) in migrate_cancel and migrate-pause for
> > basically the same reason; I don't think we've got anything similar for
> > NBD, and we probably should have (I think I asked for it fairly
> > recently).
> 
> Yes, the migrate_cancel is an example of a more fine grained way to
> recover. I was thinking that we need an equivalent fine control knob
> for NBD too.

I feel it might be nice not to have to create so many separate knobs.

> That way if QEMU does get stuck, you can start by tearing down the
> least distruptive channel. eg try tearing down the migration connection
> first (which shouldn't negatively impact the guest), and only if that
> doesn't work then, move on to tear down the NBD connection (which risks
> data loss)

I wonder if a different way would be to make all network connections
register with yank, but then make yank take a list of connections to
shutdown(2).

Dave

> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK