linux-kernel - Re: [PATCH v11 4/6] mm: function to offer a page block on the free list

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Message-Id: <c5f8cf53-c30b-a7ec-a8e8-9a2c120bdff6@de.ibm.com>
Date:   Wed, 21 Jun 2017 14:56:03 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Hildenbrand <david@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Wei Wang <wei.w.wang@...el.com>, linux-kernel@...r.kernel.org,
        qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
        kvm@...r.kernel.org, linux-mm@...ck.org, mst@...hat.com,
        cornelia.huck@...ibm.com, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com
Cc:     Nitesh Narayan Lal <nilal@...hat.com>
Subject: Re: [PATCH v11 4/6] mm: function to offer a page block on the free
 list

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.
> 
Two reasons why I disagree:
- virtio-balloon is often used as memory hotplug. (e.g. libvirts current/max memory
uses virtio ballon)
- free page hinting will not allow to shrink the page cache of guests (like a ballooner does)