On Wed, Nov 28, 2012 at 08:18:01PM -0800, John Stultz wrote:

On 11/21/2012 04:36 PM, John Stultz wrote:

quoted

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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