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

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Message-ID: <1497977049.20270.100.camel@redhat.com>
Date:   Tue, 20 Jun 2017 12:44:09 -0400
From:   Rik van Riel <riel@...hat.com>
To:     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,
        david@...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 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?

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