linux-kernel - Re: [PATCH] xen/balloon: add late_initcall

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Date:   Fri, 29 Oct 2021 12:22:18 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] xen/balloon: add late_initcall_sync() for initial
 ballooning done

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@...r.kernel.org>
>>>> Reported-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
>>>> Signed-off-by: Juergen Gross <jgross@...e.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.

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().

What do you think?


Juergen


Juergen

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