Re: [PATCH] xen/balloon: add late_initcall_sync() for initial ballooning done

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Fri, Oct 29, 2021 at 12:22:18PM +0200, Juergen Gross wrote:
> On 29.10.21 11:57, Marek Marczykowski-Górecki wrote:
> > On Fri, Oct 29, 2021 at 06:48:44AM +0200, Juergen Gross wrote:
> > > On 28.10.21 22:16, Marek Marczykowski-Górecki wrote:
> > > > On Thu, Oct 28, 2021 at 12:59:52PM +0200, Juergen Gross wrote:
> > > > > When running as PVH or HVM guest with actual memory < max memory the
> > > > > hypervisor is using "populate on demand" in order to allow the guest
> > > > > to balloon down from its maximum memory size. For this to work
> > > > > correctly the guest must not touch more memory pages than its target
> > > > > memory size as otherwise the PoD cache will be exhausted and the guest
> > > > > is crashed as a result of that.
> > > > > 
> > > > > In extreme cases ballooning down might not be finished today before
> > > > > the init process is started, which can consume lots of memory.
> > > > > 
> > > > > In order to avoid random boot crashes in such cases, add a late init
> > > > > call to wait for ballooning down having finished for PVH/HVM guests.
> > > > > 
> > > > > Cc: <stable@vger.kernel.org>
> > > > > Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> > > > > Signed-off-by: Juergen Gross <jgross@suse.com>
> > > > 
> > > > It may happen that initial balloon down fails (state==BP_ECANCELED). In
> > > > that case, it waits indefinitely. I think it should rather report a
> > > > failure (and panic? it's similar to OOM before PID 1 starts, so rather
> > > > hard to recover), instead of hanging.
> > > 
> > > Okay, I can add something like that. I'm thinking of issuing a failure
> > > message in case of credit not having changed for 1 minute and panic()
> > > after two more minutes. Is this fine?
> > 
> > Isn't it better to get a state from balloon_thread()? If the balloon
> > fails it won't really try anymore (until 3600s timeout), so waiting in
> > that state doesn't help. And reporting the failure earlier may be more
> > user friendly. Or maybe there is something that could wakeup the thread
> > earlier, that I don't see? Hot plugging more RAM is rather unlikely at
> > this stage...
> 
> Waking up the thread would be easy, but probably that wouldn't really
> help.

Waking it up alone no. I was thinking what could wake it up - if
nothing, then definitely waiting wouldn't help. You explained that just
below:

> The idea was that maybe a Xen admin would see the guest not booting up
> further and then adding some more memory to the guest (this should wake
> up the balloon thread again).
> 
> I agree that stopping to wait for ballooning to finish in case of it
> having failed is probably a sensible thing to do. Additionally I could
> add a boot parameter to control the timeout after the fail message and
> the panic().

Right, that would make sense: it's basically a time admin has to plug in
more memory to the VM.

> What do you think?

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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