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

From	David Hildenbrand <david@redhat.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v11 4/6] mm: function to offer a page block on the free list
Date	2017-06-21 15:50 +0200
Message-ID	<tUOb8-6Jf-11@gated-at.bofh.it> (permalink)
References	(1 earlier) <tQpEl-qv-7@gated-at.bofh.it> <tRyme-3aL-9@gated-at.bofh.it> <tUuvM-2D0-1@gated-at.bofh.it> <tUuvM-2D0-15@gated-at.bofh.it> <tUNoK-6da-19@gated-at.bofh.it>
Organization	Red Hat GmbH

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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

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Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list Rik van Riel <riel@redhat.com> - 2017-06-20 18:50 +0200
  Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list David Hildenbrand <david@redhat.com> - 2017-06-20 18:50 +0200
    Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list Rik van Riel <riel@redhat.com> - 2017-06-20 19:30 +0200
      Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list "Michael S. Tsirkin" <mst@redhat.com> - 2017-06-20 20:30 +0200
        Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list Rik van Riel <riel@redhat.com> - 2017-06-20 22:00 +0200
          Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list "Michael S. Tsirkin" <mst@redhat.com> - 2017-06-21 14:50 +0200
      Re: [Qemu-devel] [PATCH v11 4/6] mm: function to offer a page block  on the free list Wei Wang <wei.w.wang@intel.com> - 2017-06-21 10:40 +0200
    Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list "Michael S. Tsirkin" <mst@redhat.com> - 2017-06-20 20:20 +0200
      Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list "Michael S. Tsirkin" <mst@redhat.com> - 2017-06-20 21:00 +0200
        Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list David Hildenbrand <david@redhat.com> - 2017-06-20 21:10 +0200
      Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list David Hildenbrand <david@redhat.com> - 2017-06-20 21:00 +0200
    Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list Christian Borntraeger <borntraeger@de.ibm.com> - 2017-06-21 15:00 +0200
      Re: [PATCH v11 4/6] mm: function to offer a page block on the free  list David Hildenbrand <david@redhat.com> - 2017-06-21 15:50 +0200

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