Re: Can get_user_pages( ,write=1, force=1, ) result in a read-only pte and _count=2?

[Robin Holt <holt@sgi.com>]

On Mon, Jun 23, 2008 at 05:48:17PM +0100, Hugh Dickins wrote:
> On Mon, 23 Jun 2008, Robin Holt wrote:
> > All that said, I think the race we discussed earlier in the thread is
> > a legitimate one and believe Hugh's fix is correct.
> 
> My fix?  Would that be the get_user_pages VM_WRITE test before clearing
> FOLL_WRITE - which I believe didn't fix you at all?  Or the grand new

It did not fix the problem I was seeing, but I believe it is a possible
race condition.  I certainly admit to not having a complete enough
understanding and there may be something which prevents that from being
a problem, but I currently still think there is a problem, just not one
I can reproduce.

> reuse test in do_wp_page that I'm still working on - of which Nick sent
> a lock_page approximation for you to try?  Would you still be able to
> try mine when I'm ready, or does it now appear irrelevant to you?

Before your response, I had convinced myself my problem was specific to
XPMEM, but I see your point and may agree that it is a problem for all
get_user_pages() users.

I can certainly test when you have it ready.

I had confused myself about Nick's first patch.  I will give that
another look over and see if it fixes the problem.

> 	http://lkml.org/lkml/2006/9/14/384
> 
> but it's a broken thread, with misunderstanding on all sides,
> so rather hard to get a grasp of it.

That is extremely similar to the issue I am seeing.  I think that if
Infiniband were using the mmu_notifier stuff, they would be closer, but
IIRC, there are significant hardware restrictions which prevent demand
paging for working on some IB devices.

Thanks,
Robin

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