Re: [PATCH] KVM: arm/arm64: Skip updating page table entry if no change

[Punit Agrawal]

Marc Zyngier <marc.zyngier@xxxxxxx> writes:

> Hi Punit,
>
> On Fri, 10 Aug 2018 12:13:00 +0100,
> Punit Agrawal <punit.agrawal@xxxxxxx> wrote:
>> 
>> Contention on updating a page table 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.
>> 
>> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
>> Cc: Suzuki Poulose <suzuki.poulose@xxxxxxx>
>
> This definitely deserves a Cc to stable, and a Fixes: tag.

Agreed.

For PMD the issue exists since commit ad361f093c1e ("KVM: ARM: Support
hugetlbfs backed huge pages") when the file lived in arch/arm/kvm. (v3.12+)

For PTE the issue exists since commit d5d8184d35c9 ("KVM: ARM: Memory
virtualization setup"). (v3.8+)

I'll split the fix into two patches and add a cc to stable.

>> --
>> Hi,
>> 
>> This problem was reported by a user when testing PUD hugepages. During
>> VM restore when all threads are running cpu intensive workload, the
>> refauting was causing the VM to not make any forward progress.
>> 
>> This patch fixes the problem for PMD and PTE page fault handling.
>> 
>> Thanks,
>> Punit
>> 
>> Change-Id: I04c9aa8b9fbada47deb1a171c9959f400a0d2a21

Just noticed this. Looks like the commit hook has escaped it's worktree.

>> ---
>>  virt/kvm/arm/mmu.c | 16 ++++++++++++++++
>>  1 file changed, 16 insertions(+)
>> 
>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>> index 1d90d79706bd..a66a5441ca2f 100644
>> --- a/virt/kvm/arm/mmu.c
>> +++ b/virt/kvm/arm/mmu.c
>> @@ -1027,6 +1027,18 @@ static int stage2_set_pmd_huge(struct kvm *kvm, struct kvm_mmu_memory_cache
>>  	VM_BUG_ON(pmd_present(*pmd) && pmd_pfn(*pmd) != pmd_pfn(*new_pmd));
>>  
>>  	old_pmd = *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))
>> +		return 0;
>
> Shouldn't you take this opportunity to also refactor it with the above
> VM_BUG_ON and the below pmd_present? At the moment, we end-up testing
> pmd_present twice, and your patch is awfully similar to the VM_BUG_ON
> one.

I went for the minimal change keeping a backport in mind.

The VM_BUG_ON() is enabled only when CONFIG_DEBUG_VM is selected and
checks for pfn ignoring the attributes, while this fix checks for the
entire entry. Maybe I'm missing something, but I can't see an obvious
way to combine the checks.

I've eliminated the duplicate pmd_present() check and will post the
updated patches.

Thanks,
Punit

>> +
>>  	if (pmd_present(old_pmd)) {
>>  		pmd_clear(pmd);
>>  		kvm_tlb_flush_vmid_ipa(kvm, addr);
>> @@ -1101,6 +1113,10 @@ static int stage2_set_pte(struct kvm *kvm, struct kvm_mmu_memory_cache *cache,
>>  
>>  	/* Create 2nd stage page table mapping - Level 3 */
>>  	old_pte = *pte;
>> +	/* Skip page table update if there is no change */
>> +	if (pte_val(old_pte) == pte_val(*new_pte))
>> +		return 0;
>> +
>>  	if (pte_present(old_pte)) {
>>  		kvm_set_pte(pte, __pte(0));
>>  		kvm_tlb_flush_vmid_ipa(kvm, addr);
>
> Thanks,
>
> 	M.