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

From: Minchan Kim
Date: Mon Dec 03 2012 - 19:00:01 EST

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.
> 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
> -john
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Kind regards,
Minchan Kim
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