linux-kernel - Re: [PATCH] Add a page cache-backed balloon device driver.

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Message-ID: <CAK+C7kWKtoYC=1fBQ45RTFfNc3p2PwZbLMOefMTtzOfsqe_k8w@mail.gmail.com>
Date:	Tue, 26 Jun 2012 14:31:26 -0700
From:	Frank Swiderski <fes@...gle.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	mikew@...gle.com, Ying Han <yinghan@...gle.com>,
	Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH] Add a page cache-backed balloon device driver.

On Tue, Jun 26, 2012 at 1:40 PM, Rik van Riel <riel@...hat.com> wrote:
> On 06/26/2012 04:32 PM, Frank Swiderski wrote:
>>
>> This implementation of a virtio balloon driver uses the page cache to
>> "store" pages that have been released to the host.  The communication
>> (outside of target counts) is one way--the guest notifies the host when
>> it adds a page to the page cache, allowing the host to madvise(2) with
>> MADV_DONTNEED.  Reclaim in the guest is therefore automatic and implicit
>> (via the regular page reclaim).  This means that inflating the balloon
>> is similar to the existing balloon mechanism, but the deflate is
>> different--it re-uses existing Linux kernel functionality to
>> automatically reclaim.
>>
>> Signed-off-by: Frank Swiderski<fes@...gle.com>
>
>
> It is a great idea, but how can this memory balancing
> possibly work if someone uses memory cgroups inside a
> guest?

Thanks and good point--this isn't something that I considered in the
implementation.

> Having said that, we currently do not have proper
> memory reclaim balancing between cgroups at all, so
> requiring that of this balloon driver would be
> unreasonable.
>
> The code looks good to me, my only worry is the
> code duplication. We now have 5 balloon drivers,
> for 4 hypervisors, all implementing everything
> from scratch...

Do you have any recommendations on this?  I could (I think reasonably
so) modify the existing virtio_balloon.c and have it change behavior
based on a feature bit or other configuration.  I'm not sure that
really addresses the root of what you're pointing out--it's still
adding a different implementation, but doing so as an extension of an
existing one.

fes
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