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

[Jerome Glisse <j.glisse@gmail.com>]

On Wed, Jan 22, 2014 at 04:01:15PM +0200, Haggai Eran wrote:
> On 22/01/2014 15:10, Andrea Arcangeli wrote:
> > On Wed, Jan 15, 2014 at 11:40:34AM +0200, Mike Rapoport wrote:
> >> Commit 6bdb913f0a70a4dfb7f066fb15e2d6f960701d00 (mm: wrap calls to
> >> set_pte_at_notify with invalidate_range_start and invalidate_range_end)
> >> breaks semantics of set_pte_at_notify. When calls to set_pte_at_notify
> >> are wrapped with mmu_notifier_invalidate_range_start and
> >> mmu_notifier_invalidate_range_end, KVM zaps pte during
> >> mmu_notifier_invalidate_range_start callback and set_pte_at_notify has
> >> no spte to update and therefore it's called for nothing.
> >>
> >> As Andrea suggested (1), the problem is resolved by calling
> >> mmu_notifier_invalidate_page after PT lock has been released and only
> >> for mmu_notifiers that do not implement change_ptr callback.
> >>
> >> (1) http://thread.gmane.org/gmane.linux.kernel.mm/111710/focus=111711
> >>
> >> Reported-by: Izik Eidus <izik.eidus@ravellosystems.com>
> >> Signed-off-by: Mike Rapoport <mike.rapoport@ravellosystems.com>
> >> Cc: Andrea Arcangeli <aarcange@redhat.com>
> >> Cc: Haggai Eran <haggaie@mellanox.com>
> >> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >> ---
> >>  include/linux/mmu_notifier.h | 31 ++++++++++++++++++++++++++-----
> >>  kernel/events/uprobes.c      | 12 ++++++------
> >>  mm/ksm.c                     | 15 +++++----------
> >>  mm/memory.c                  | 14 +++++---------
> >>  mm/mmu_notifier.c            | 24 ++++++++++++++++++++++--
> >>  5 files changed, 64 insertions(+), 32 deletions(-)
> > 
> > Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> > 
> 
> Hi Andrea, Mike,
> 
> Did you get a chance to consider the scenario I wrote about in the other
> thread?
> 
> 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.

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.

Cheers,
Jerome


> Regards,
> Haggai
> 
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