From: Jérôme Glisse <[email protected]> This patchset is on top of my patchset to add context information to mmu notifier [1] you can find a branch with everything [2]. I have not tested it but i wanted to get the discussion started. I believe it is correct but i am not sure what kind of kvm test i can run to exercise this.
The idea is that since kvm will invalidate the secondary MMUs within invalidate_range callback then the change_pte() optimization is lost. With this patchset everytime core mm is using set_pte_at_notify() and thus change_pte() get calls then we can ignore the invalidate_range callback altogether and only rely on change_pte callback. Note that this is only valid when either going from a read and write pte to a read only pte with same pfn, or from a read only pte to a read and write pte with different pfn. The other side of the story is that the primary mmu pte is clear with ptep_clear_flush_notify before the call to change_pte. Also with the mmu notifier context information [1] you can further optimize other cases like mprotect or write protect when forking. You can use the new context information to infer that the invalidation is for read only update of the primary mmu and update the secondary mmu accordingly instead of clearing it and forcing fault even for read access. I do not know if that is an optimization that would bear any fruit for kvm. It does help for device driver. You can also optimize the soft dirty update. Cheers, Jérôme [1] https://lore.kernel.org/linux-fsdevel/[email protected]/T/#m69e8f589240e18acbf196a1c8aa1d6fc97bd3565 [2] https://cgit.freedesktop.org/~glisse/linux/log/?h=kvm-restore-change_pte Cc: Andrea Arcangeli <[email protected]> Cc: Peter Xu <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Arnaldo Carvalho de Melo <[email protected]> Cc: Alexander Shishkin <[email protected]> Cc: Jiri Olsa <[email protected]> Cc: Namhyung Kim <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Matthew Wilcox <[email protected]> Cc: Paolo Bonzini <[email protected]> Cc: Radim Krčmář <[email protected]> Cc: Michal Hocko <[email protected]> Cc: [email protected] Jérôme Glisse (4): uprobes: use set_pte_at() not set_pte_at_notify() mm/mmu_notifier: use unsigned for event field in range struct mm/mmu_notifier: set MMU_NOTIFIER_USE_CHANGE_PTE flag where appropriate kvm/mmu_notifier: re-enable the change_pte() optimization. include/linux/mmu_notifier.h | 21 +++++++++++++++++++-- kernel/events/uprobes.c | 3 +-- mm/ksm.c | 6 ++++-- mm/memory.c | 3 ++- virt/kvm/kvm_main.c | 16 ++++++++++++++++ 5 files changed, 42 insertions(+), 7 deletions(-) -- 2.17.1
