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

From: John Stultz
Date: Tue Dec 04 2012 - 14:14:44 EST

On 12/03/2012 11:22 PM, Minchan Kim wrote:
On Mon, Dec 03, 2012 at 04:57:20PM -0800, John Stultz wrote:
On 12/03/2012 04:00 PM, Minchan Kim wrote:
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
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?
Yes, depending on the pattern that MADV_VOLATILE and MADV_NOVOLATILE
is applied, we can get a result where data is purged, but we aren't
notified of it. Also, since madvise returns early if it encounters
an error, in the case where you have checkerboard volatile regions
(say every other page is volatile), which you mark non-volatile with
one large MADV_NOVOLATILE call, the first volatile vma will be
marked non-volatile, but since it returns purged, the madvise loop
will stop and the following volatile regions will be left volatile.

The patches in the git tree below which handle the perged state
better seem to work for my tests, as far as resolving any
overlapping calls. Of course there may yet still be problems I've
not found.

Anyway, take a look at the tree here and let me know what you think.;a=shortlog;h=refs/heads/dev/minchan-anonvol
Eager to hear what you think!
Below two patches look good to me.

[rmap: Simplify volatility checking by moving it out of try_to_unmap_one]
[rmap: ClearPageDirty() when returning SWAP_DISCARD]

[madvise: Fix NOVOLATILE bug]
I can't understand description of the patch.
Could you elaborate it with example?
The case I ran into here is if you have a range where you mark every other page as volatile. Then mark all the pages in that range as non-volatile in one madvise call.

sys_madvise() will then find the first vma in the range, and call madvise_vma(), which marks the first vma non-volatile and return the purged state. If the page has been purged, sys_madvise code will note that as an error, and break out of the vma iteration loop, leaving the following vmas in the range volatile.

[madvise: Fixup vma->purged handling]
I included VM_VOLATILE into VM_SPECIAL intentionally.
If comment of VM_SPECIAL is right, merge with volatile vmas shouldn't happen.
So I guess you see other problem. When I see my source code today, locking
scheme/purge handling is totally broken. I will look at it. Maybe you are seeing
bug related that. Part of patch is needed. It could be separate patch.
I will merge it.
I don't think the problem is when vmas being marked VM_VOLATILE are being merged, its that when we mark the vma as *non-volatile*, and remove the VM_VOLATILE flag we merge the non-volatile vmas with neighboring vmas. So preserving the purged flag during that merge is important. Again, the example I used to trigger this was an alternating pattern of volatile and non volatile vmas, then marking the entire range non-volatile (though sometimes in two overlapping passes).


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