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

[Minchan Kim]

On Fri, Dec 07, 2012 at 04:49:56PM -0800, John Stultz wrote:
> On 12/04/2012 08:18 PM, Minchan Kim wrote:
> >On Tue, Dec 04, 2012 at 11:13:40AM -0800, John Stultz wrote:
> >>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).
> >Understood. Thanks.
> >Below patch solves your problems? It's simple than yours.
> 
> Yea, this is nicer then my fix.
> Although I still need the purged handling in the vma merge code for
> me to see the behavior I expect in my tests.
> 
> I've integrated your patch and repushed my queue here:
> http://git.linaro.org/gitweb?p=people/jstultz/android-dev.git;a=shortlog;h=refs/heads/dev/minchan-anonvol
> 
> git://git.linaro.org/people/jstultz/android-dev.git dev/minchan-anonvol
> 
> >Anyway, both yours and mine are not right fix.
> >As I mentioned, locking scheme is broken.
> >We need anon_vma_lock to handle purged and we should consider fork
> >case, too.
> Hrm. I'm sure you're right, as I've not yet fully grasped all the
> locking rules here.  Could you clarify how it is broken? And why is
> the anon_vma_lock needed to manage the purged state that is part of
> the vma itself?

If you don't hold anon->lock, merge/split/fork can race with try_to_unmap
so vma->[purged|vm_flags] would lose consistency.

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