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

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Date:   Fri, 29 Oct 2021 06:48:44 +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 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?


Juergen

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