linux-kernel - Re: [PATCH v4] xen/balloon: add late_initcall

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Date:   Thu, 4 Nov 2021 17:44:56 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Jonathan Corbet <corbet@....net>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Subject: Re: [PATCH v4] xen/balloon: add late_initcall_sync() for initial
 ballooning done

On 04.11.21 17:34, Boris Ostrovsky wrote:
> 
> On 11/4/21 12:21 PM, Juergen Gross wrote:
>> On 04.11.21 16:55, Boris Ostrovsky wrote:
>>>
>>> On 11/3/21 9:55 PM, Boris Ostrovsky wrote:
>>>>
>>>> On 11/2/21 5:19 AM, 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.
>>>>>
>>>>> Warn on console if initial ballooning fails, panic() after stalling
>>>>> for more than 3 minutes per default. Add a module parameter for
>>>>> changing this timeout.
>>>>>
>>>>> Cc: <stable@...r.kernel.org>
>>>>> Reported-by: Marek Marczykowski-Górecki 
>>>>> <marmarek@...isiblethingslab.com>
>>>>> Signed-off-by: Juergen Gross <jgross@...e.com>
>>>>
>>>>
>>>>
>>>> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>>>
>>>
>>> This appears to have noticeable effect on boot time (and boot 
>>> experience in general).
>>>
>>>
>>> I have
>>>
>>>
>>>    memory=1024
>>>    maxmem=8192
>>>
>>>
>>> And my boot time (on an admittedly slow box) went from 33 to 45 
>>> seconds. And boot pauses in the middle while it is waiting for 
>>> ballooning to complete.
>>>
>>>
>>> [    5.062714] xen:balloon: Waiting for initial ballooning down 
>>> having finished.
>>> [    5.449696] random: crng init done
>>> [   34.613050] xen:balloon: Initial ballooning down finished.
>>
>> This shows that before it was just by chance that the PoD cache wasn't
>> exhausted.
> 
> 
> True.
> 
> 
>>
>>> So at least I think we should consider bumping log level down from info.
>>
>> Which level would you prefer? warn?
>>
> 
> Notice? Although that won't make much difference as WARN is the default 
> level.

Right. That was my thinking.

> I suppose we can't turn scrubbing off at this point?

I don't think we can be sure a ballooned page wasn't in use before. And
it could contain some data e.g. from the loaded initrd, maybe even put
there by the boot loader. So no, I wouldn't want to do that by default.

We could add another value to the xen_scrub_pages boot parameter, like
xen_scrub_pages=not-at-boot or some such. But this should be another
patch. And it should be documented that initrd or kernel data might
leak.

>> And if so, would you mind doing this while committing (I have one day
>> off tomorrow)?
> 
> 
> Yes, of course.

Thanks.


Juergen


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