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

Punit Agrawal
2018-08-13T10:01:27Z

Suzuki K Poulose <suzuki.poulose@arm.com> writes:

> On 08/13/2018 10:40 AM, 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;
>
> minor nit: You could as well return here, as there are no other users
> for the label and there are no clean up actions.

Ok - I'll do a quick respin for the maintainers to pick up if they are
happy with the other aspects of the patch.

>
> Either way,
>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Thanks Suzuki.

>
>
>> +
>> 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;
>> }
>>
>>
>
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