[PATCH v2 1/2] KVM: arm/arm64: Skip updating PMD entry if no change

Marc Zyngier
2018-08-13T10:11:30Z

Hi Punit,

On 13/08/18 10:40, Punit Agrawal wrote:
> Contention on updating a PMD entry by a large number of vcpus can lead
> to duplicate work when handling stage 2 page faults. As the page table
> update follows the break-before-make requirement of the architecture,
> it can lead to repeated refaults due to clearing the entry and
> flushing the tlbs.
>
> This problem is more likely when -
>
> * there are large number of vcpus
> * the mapping is large block mapping
>
> such as when using PMD hugepages (512MB) with 64k pages.
>
> Fix this by skipping the page table update if there is no change in
> the entry being updated.
>
> Fixes: ad361f093c1e ("KVM: ARM: Support hugetlbfs backed huge pages")
> Change-Id: Ib417957c842ef67a6f4b786f68df62048d202c24
> Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: Suzuki Poulose <suzuki.poulose@arm.com>
> Cc: stable at vger.kernel.org
> ---
> virt/kvm/arm/mmu.c | 40 +++++++++++++++++++++++++++++-----------
> 1 file changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 1d90d79706bd..2ab977edc63c 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1015,19 +1015,36 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> pmd = stage2_get_pmd(kvm, cache, addr);
> VM_BUG_ON(!pmd);
>
> - /*
> - * Mapping in huge pages should only happen through a fault. If a
> - * page is merged into a transparent huge page, the individual
> - * subpages of that huge page should be unmapped through MMU
> - * notifiers before we get here.
> - *
> - * Merging of CompoundPages is not supported; they should become
> - * splitting first, unmapped, merged, and mapped back in on-demand.
> - */
> - VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
> -
> old_pmd = *pmd;
> +
> if (pmd_present(old_pmd)) {
> + /*
> + * Mapping in huge pages should only happen through a
> + * fault. If a page is merged into a transparent huge
> + * page, the individual subpages of that huge page
> + * should be unmapped through MMU notifiers before we
> + * get here.
> + *
> + * Merging of CompoundPages is not supported; they
> + * should become splitting first, unmapped, merged,
> + * and mapped back in on-demand.
> + */
> + VM_BUG_ON(pmd_pfn(old_pmd) != pmd_pfn(*new_pmd));
> +
> + /*
> + * Multiple vcpus faulting on the same PMD entry, can
> + * lead to them sequentially updating the PMD with the
> + * same value. Following the break-before-make
> + * (pmd_clear() followed by tlb_flush()) process can
> + * hinder forward progress due to refaults generated
> + * on missing translations.
> + *
> + * Skip updating the page table if the entry is
> + * unchanged.
> + */
> + if (pmd_val(old_pmd) == pmd_val(*new_pmd))
> + goto out;

I think the order of these two checks should be reversed: the first one
is clearly a subset of the second one, so it'd make sense to have the
global comparison before having the more specific one. Not that it
matter much in practice, but I just find it easier to reason about.

> +
> pmd_clear(pmd);
> kvm_tlb_flush_vmid_ipa(kvm, addr);
> } else {
> @@ -1035,6 +1052,7 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
> }
>
> kvm_set_pmd(pmd, *new_pmd);
> +out:
> return 0;
> }
>
>

Thanks,

M.
--
Jazz is not dead. It just smells funny...