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Date:   Fri, 29 Oct 2021 11:57:17 +0200
From:   Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= 
        <marmarek@invisiblethingslab.com>
To:     Juergen Gross <jgross@suse.com>
Cc:     xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org
Subject: Re: [PATCH] xen/balloon: add late_initcall_sync() for initial
 ballooning done
Message-ID: <YXvFfRKuD574hulr@mail-itl>
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Date: Fri, 29 Oct 2021 11:57:17 +0200
From: Marek =?utf-8?Q?Marczykowski-G=C3=B3recki?= <marmarek@invisiblethingslab.com>
To: Juergen Gross <jgross@suse.com>
Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org
Subject: Re: [PATCH] xen/balloon: add late_initcall_sync() for initial
 ballooning done

On Fri, Oct 29, 2021 at 06:48:44AM +0200, Juergen Gross wrote:
> On 28.10.21 22:16, Marek Marczykowski-G=C3=B3recki 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.
> > >=20
> > > In extreme cases ballooning down might not be finished today before
> > > the init process is started, which can consume lots of memory.
> > >=20
> > > 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.
> > >=20
> > > Cc: <stable@vger.kernel.org>
> > > Reported-by: Marek Marczykowski-G=C3=B3recki <marmarek@invisiblething=
slab.com>
> > > Signed-off-by: Juergen Gross <jgross@suse.com>
> >=20
> > It may happen that initial balloon down fails (state=3D=3DBP_ECANCELED)=
=2E 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.
>=20
> 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...
See my patch at [1], although rather hacky (and likely - racy).

[1] https://lore.kernel.org/xen-devel/YXFxKC4shCATB913@mail-itl/

--=20
Best Regards,
Marek Marczykowski-G=C3=B3recki
Invisible Things Lab

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