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Subject: Re: [PATCH v11 4/6] mm: function to offer a page block on the free
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To: Christian Borntraeger <borntraeger@de.ibm.com>,
        Rik van Riel <riel@redhat.com>, Dave Hansen <dave.hansen@intel.com>,
        Wei Wang <wei.w.wang@intel.com>, linux-kernel@vger.kernel.org,
        qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org,
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        cornelia.huck@de.ibm.com, akpm@linux-foundation.org,
        mgorman@techsingularity.net, aarcange@redhat.com, amit.shah@redhat.com,
        pbonzini@redhat.com, liliang.opensource@gmail.com
Cc: Nitesh Narayan Lal <nilal@redhat.com>
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 <c5f8cf53-c30b-a7ec-a8e8-9a2c120bdff6@de.ibm.com>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <a4f68630-c143-2579-8999-b1506bc6e272@redhat.com>
Date: Wed, 21 Jun 2017 15:47:15 +0200
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On 21.06.2017 14:56, Christian Borntraeger wrote:
> On 06/20/2017 06:49 PM, David Hildenbrand wrote:
>> On 20.06.2017 18:44, Rik van Riel wrote:
>>> On Mon, 2017-06-12 at 07:10 -0700, Dave Hansen wrote:
>>>
>>>> The hypervisor is going to throw away the contents of these pages,
>>>> right?  As soon as the spinlock is released, someone can allocate a
>>>> page, and put good data in it.  What keeps the hypervisor from
>>>> throwing
>>>> away good data?
>>>
>>> That looks like it may be the wrong API, then?
>>>
>>> We already have hooks called arch_free_page and
>>> arch_alloc_page in the VM, which are called when
>>> pages are freed, and allocated, respectively.
>>>
>>> Nitesh Lal (on the CC list) is working on a way
>>> to efficiently batch recently freed pages for
>>> free page hinting to the hypervisor.
>>>
>>> If that is done efficiently enough (eg. with
>>> MADV_FREE on the hypervisor side for lazy freeing,
>>> and lazy later re-use of the pages), do we still
>>> need the harder to use batch interface from this
>>> patch?
>>>
>> David's opinion incoming:
>>
>> No, I think proper free page hinting would be the optimum solution, if
>> done right. This would avoid the batch interface and even turn
>> virtio-balloon in some sense useless.
>>

I said "some sense" for a reason. Mainly because other techniques are
being worked on that are to fill the holes.

> Two reasons why I disagree:
> - virtio-balloon is often used as memory hotplug. (e.g. libvirts current/max memory
> uses virtio ballon)

I know, while one can argue if this real unplug as there are basically
no guarantees (see virtio-mem RFC) it is used by people because there is
simply no alternative. Still, for now some people use it for that.

> - free page hinting will not allow to shrink the page cache of guests (like a ballooner does)

There are currently some projects ongoing that try to avoid the page
cache in the guest completely.


-- 

Thanks,

David