Re: [PATCH] mm/mmu_notifier: restore set_pte_at_notify semantics

[Haggai Eran]

On 03/30/2014 11:33 PM, Jerome Glisse wrote:
> On Wed, Jan 22, 2014 at 04:01:15PM +0200, Haggai Eran wrote:
> > I'm worried about the following scenario:
> >
> > Given a read-only page, suppose one host thread (thread 1) writes to
> > that page, and performs COW, but before it calls the
> > mmu_notifier_invalidate_page_if_missing_change_pte function another host
> > thread (thread 2) writes to the same page (this time without a page
> > fault). Then we have a valid entry in the secondary page table to a
> > stale page, and someone (thread 3) may read stale data from there.
> >
> > Here's a diagram that shows this scenario:
> >
> > Thread 1                                | Thread 2        | Thread 3
> > ========================================================================
> > do_wp_page(page 1)                      |                 |
> >    ...                                   |                 |
> >    set_pte_at_notify                     |                 |
> >    ...                                   | write to page 1 |
> >                                          |                 | stale access
> >    pte_unmap_unlock                      |                 |
> >    invalidate_page_if_missing_change_pte |                 |
> >
> > This is currently prevented by the use of the range start and range end
> > notifiers.
> >
> > Do you agree that this scenario is possible with the new patch, or am I
> > missing something?
> I believe you are right, but of all the upstream user of the mmu_notifier
> API only xen would suffer from this ie any user that do not have a proper
> change_pte callback can see the bogus scenario you describe above.
Yes. I sent our RDMA paging RFC patch-set on linux-rdma [1] last month, 
and it would also suffer from this scenario, but it's not upstream yet.
> The issue i see is with user that want to/or might sleep when they are
> invalidation the secondary page table. The issue being that change_pte is
> call with the cpu page table locked (well at least for the affected pmd).
>
> I would rather keep the invalidate_range_start/end bracket around change_pte
> and invalidate page. I think we can fix the kvm regression by other means.
Perhaps another possibility would be to do the 
invalidate_range_start/end bracket only when the mmu_notifier is missing 
a change_pte implementation.

Best regards,
Haggai

[1] http://www.spinics.net/lists/linux-rdma/msg18906.html
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