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

[Minchan Kim]

Hi John,

On Tue, Dec 04, 2012 at 11:13:40AM -0800, John Stultz wrote:
> 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
> >>>>>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?
> >>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.
> >>>>http://git.linaro.org/gitweb?p=people/jstultz/android-dev.git;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).

If I understand correctly, you mean following as.

chunk1 = mmap(8M)
chunk2 = chunk1 + 2M;
chunk3 = chunk2 + 2M
chunk4 = chunk3 + 2M

madvise(chunk1, 2M, VOLATILE);
madvise(chunk4, 2M, VOLATILE);

/*
 * V : volatile vma
 * N : non volatile vma
 * So Now vma is VNVN.
 */
And chunk4 is purged.

int ret = madvise(chunk1, 8M, NOVOLATILE);
ASSERT(ret == 1);
/* And you expect VNVN->N ?*/

Right?
If so, why should non-volatile function semantic allow it which cross over
non-volatile areas in a range? I would like to fail such case because
in case of MADV_REMOVE, it fails in the middle of operation if it encounter
VM_LOCKED.

What do you think about it?

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