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

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Date:   Tue, 20 Jun 2017 21:17:26 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     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, 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, Nitesh Narayan Lal <nilal@...hat.com>
Subject: Re: [PATCH v11 4/6] mm: function to offer a page block on the free
 list

On Tue, Jun 20, 2017 at 06:49:33PM +0200, 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 agree generally. But we have to balance that against the fact that
this was discussed since at least 2011 and no one built this solution
yet.

> -- 
> 
> Thanks,
> 
> David