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Subject: Re: [PATCH v11 4/6] mm: function to offer a page block on the free
 list
From: Rik van Riel <riel@redhat.com>
To: David Hildenbrand <david@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, kvm@vger.kernel.org,
        linux-mm@kvack.org, mst@redhat.com, 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>
Date: Tue, 20 Jun 2017 13:29:00 -0400
In-Reply-To: <7b626551-6d1b-c8d5-4ef7-e357399e78dc@redhat.com>
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On Tue, 2017-06-20 at 18:49 +0200, David Hildenbrand wrote:
> On 20.06.2017 18:44, Rik van Riel wrote:

> > Nitesh Lal (on the CC list) is working on a way
> > to efficiently batch recently freed pages for
> > free page hinting to the hypervisor.
> >=20
> > 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?
> >=20
>=20
> David's opinion incoming:
>=20
> 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 agree with that.  Let me go into some more detail of
what Nitesh is implementing:

1) In arch_free_page, the being-freed page is added
   to a per-cpu set of freed pages.
2) Once that set is full, arch_free_pages goes into a
   slow path, which:
   2a) Iterates over the set of freed pages, and
   2b) Checks whether they are still free, and
   2c) Adds the still free pages to a list that is
       to be passed to the hypervisor, to be MADV_FREEd.
   2d) Makes that hypercall.

Meanwhile all arch_alloc_pages has to do is make sure it
does not allocate a page while it is currently being
MADV_FREEd on the hypervisor side.

The code Wei is working on looks like it could be=20
suitable for steps (2c) and (2d) above. Nitesh already
has code for steps 1 through 2b.

--=20
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