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To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        xen-devel@lists.xenproject.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org,
        =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= 
        <marmarek@invisiblethingslab.com>
References: <20211102091944.17487-1-jgross@suse.com>
 <f986a7c4-3032-fecd-2e19-4d63a2ee10c7@oracle.com>
 <f8e52acc-2566-1ed0-d2a3-21e2d468fab7@oracle.com>
 <3b1f1771-0a96-1f71-9c9d-9fb1a53a266e@suse.com>
 <18c12ead-ddf1-9231-7f3b-aafddd349dcf@oracle.com>
From:   Juergen Gross <jgross@suse.com>
Subject: Re: [PATCH v4] xen/balloon: add late_initcall_sync() for initial
 ballooning done
Message-ID: <2f3addff-fbe0-8ef0-6407-e879c0e9827f@suse.com>
Date:   Thu, 4 Nov 2021 17:44:56 +0100
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From: Juergen Gross <jgross@suse.com>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 xen-devel@lists.xenproject.org, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>,
 Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org,
 =?UTF-8?Q?Marek_Marczykowski-G=c3=b3recki?= <marmarek@invisiblethingslab.com>
Message-ID: <2f3addff-fbe0-8ef0-6407-e879c0e9827f@suse.com>
Subject: Re: [PATCH v4] xen/balloon: add late_initcall_sync() for initial
 ballooning done
References: <20211102091944.17487-1-jgross@suse.com>
 <f986a7c4-3032-fecd-2e19-4d63a2ee10c7@oracle.com>
 <f8e52acc-2566-1ed0-d2a3-21e2d468fab7@oracle.com>
 <3b1f1771-0a96-1f71-9c9d-9fb1a53a266e@suse.com>
 <18c12ead-ddf1-9231-7f3b-aafddd349dcf@oracle.com>
In-Reply-To: <18c12ead-ddf1-9231-7f3b-aafddd349dcf@oracle.com>

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On 04.11.21 17:34, Boris Ostrovsky wrote:
>=20
> 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 th=
e
>>>>> hypervisor is using "populate on demand" in order to allow the gues=
t
>>>>> to balloon down from its maximum memory size. For this to work
>>>>> correctly the guest must not touch more memory pages than its targe=
t
>>>>> memory size as otherwise the PoD cache will be exhausted and the gu=
est
>>>>> 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 ini=
t
>>>>> call to wait for ballooning down having finished for PVH/HVM guests=
=2E
>>>>>
>>>>> 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@vger.kernel.org>
>>>>> Reported-by: Marek Marczykowski-G=C3=B3recki=20
>>>>> <marmarek@invisiblethingslab.com>
>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>
>>>>
>>>>
>>>> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>>
>>>
>>> This appears to have noticeable effect on boot time (and boot=20
>>> experience in general).
>>>
>>>
>>> I have
>>>
>>>
>>> =C2=A0=C2=A0 memory=3D1024
>>> =C2=A0=C2=A0 maxmem=3D8192
>>>
>>>
>>> And my boot time (on an admittedly slow box) went from 33 to 45=20
>>> seconds. And boot pauses in the middle while it is waiting for=20
>>> ballooning to complete.
>>>
>>>
>>> [=C2=A0=C2=A0=C2=A0 5.062714] xen:balloon: Waiting for initial balloo=
ning down=20
>>> having finished.
>>> [=C2=A0=C2=A0=C2=A0 5.449696] random: crng init done
>>> [=C2=A0=C2=A0 34.613050] xen:balloon: Initial ballooning down finishe=
d.
>>
>> This shows that before it was just by chance that the PoD cache wasn't=

>> exhausted.
>=20
>=20
> True.
>=20
>=20
>>
>>> So at least I think we should consider bumping log level down from in=
fo.
>>
>> Which level would you prefer? warn?
>>
>=20
> Notice? Although that won't make much difference as WARN is the default=
=20
> 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=3Dnot-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)?
>=20
>=20
> Yes, of course.

Thanks.


Juergen


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