linux-kernel - Re: [RFC v2] Support volatile range for anon vma

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Date:	Tue, 4 Dec 2012 09:00:42 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Christoph Lameter <cl@...ux.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC v2] Support volatile range for anon vma

On Wed, Nov 28, 2012 at 08:18:01PM -0800, John Stultz wrote:
> On 11/21/2012 04:36 PM, John Stultz wrote:
> >2) Being able to use this with tmpfs files. I'm currently trying
> >to better understand the rmap code, looking to see if there's a
> >way to have try_to_unmap_file() work similarly to
> >try_to_unmap_anon(), to allow allow users to madvise() on mmapped
> >tmpfs files. This would provide a very similar interface as to
> >what I've been proposing with fadvise/fallocate, but just using
> >process virtual addresses instead of (fd, offset) pairs.   The
> >benefit with (fd,offset) pairs for Android is that its easier to
> >manage shared volatile ranges between two processes that are
> >sharing data via an mmapped tmpfs file (although this actual use
> >case may be fairly rare).  I believe we should still be able to
> >rework the ashmem internals to use madvise (which would provide
> >legacy support for existing android apps), so then its just a
> >question of if we could then eventually convince Android apps to
> >use the madvise interface directly, rather then the ashmem unpin
> >ioctl.
> 
> Hey Minchan,
>     I've been playing around with your patch trying to better
> understand your approach and to extend it to support tmpfs files. In
> doing so I've found a few bugs, and have some rough fixes I wanted
> to share. There's still a few edge cases I need to deal with (the
> vma-purged flag isn't being properly handled through vma merge/split
> operations), but its starting to come along.

Hmm, my patch doesn't allow to merge volatile with another one by
inserting VM_VOLATILE into VM_SPECIAL so I guess merge isn't problem.
In case of split, __split_vma copy old vma to new vma like this

        *new = *vma;

So the problem shouldn't happen, I guess.
Did you see the real problem about that?

> 
> Anyway, take a look at the tree here and let me know what you think.
> http://git.linaro.org/gitweb?p=people/jstultz/android-dev.git;a=shortlog;h=refs/heads/dev/minchan-anonvol
> 
> I'm sure much is wrong with the tree, but with it I can now mark
> tmpfs file pages as volatile/nonvolatile and see them purged under
> pressure. Unfortunately its not limited to tmpfs, so persistent
> files will also work, but the state of the underlying files on purge
> is undefined. Hopefully I can find a way to limit it to
> non-persistent filesystems for now, and if needed find a way to
> extend it to persistent filesystems in a sane way later.

I will take a look.
Thanks.

> 
> thanks
> -john
> 
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-- 
Kind regards,
Minchan Kim
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