Messages in this thread | | | Date | Wed, 2 Apr 2014 15:52:45 +0300 | From | Haggai Eran <> | Subject | Re: [PATCH] mm/mmu_notifier: restore set_pte_at_notify semantics |
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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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