Re: [RFC PATCH] KVM: arm64: Force a PTE mapping when logging is enabled

Marc Zyngier
2019-03-05T11:51:27Z

On 05/03/2019 11:09, Zenghui Yu wrote:
> Hi Marc, Suzuki,
>
> On 2019/3/5 1:34, Marc Zyngier wrote:
>> Hi Zenghui, Suzuki,
>>
>> On 04/03/2019 17:13, Suzuki K Poulose wrote:
>>> Hi Zenghui,
>>>
>>> On Sun, Mar 03, 2019 at 11:14:38PM +0800, Zenghui Yu wrote:
>>>> I think there're still some problems in this patch... Details below.
>>>>
>>>> On Sat, Mar 2, 2019 at 11:39 AM Zenghui Yu <yuzenghui@huawei.com> wrote:
>>>>>
>>>>> The idea behind this is: we don't want to keep tracking of huge pages when
>>>>> logging_active is true, which will result in performance degradation. We
>>>>> still need to set vma_pagesize to PAGE_SIZE, so that we can make use of it
>>>>> to force a PTE mapping.
>>>
>>> Yes, you're right. We are indeed ignoring the force_pte flag.
>>>
>>>>>
>>>>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
>>>>> Cc: Punit Agrawal <punit.agrawal@arm.com>
>>>>> Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
>>>>>
>>>>> ---
>>>>> Atfer looking into https://patchwork.codeaurora.org/patch/647985/ , the
>>>>> "vma_pagesize = PAGE_SIZE" logic was not intended to be deleted. As far
>>>>> as I can tell, we used to have "hugetlb" to force the PTE mapping, but
>>>>> we have "vma_pagesize" currently instead. We should set it properly for
>>>>> performance reasons (e.g, in VM migration). Did I miss something important?
>>>>>
>>>>> ---
>>>>> virt/kvm/arm/mmu.c | 7 +++++++
>>>>> 1 file changed, 7 insertions(+)
>>>>>
>>>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>>>> index 30251e2..7d41b16 100644
>>>>> --- a/virt/kvm/arm/mmu.c
>>>>> +++ b/virt/kvm/arm/mmu.c
>>>>> @@ -1705,6 +1705,13 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>> (vma_pagesize == PUD_SIZE && kvm_stage2_has_pmd(kvm))) &&
>>>>> !force_pte) {
>>>>> gfn = (fault_ipa & huge_page_mask(hstate_vma(vma))) >> PAGE_SHIFT;
>>>>> + } else {
>>>>> + /*
>>>>> + * Fallback to PTE if it's not one of the stage2
>>>>> + * supported hugepage sizes or the corresponding level
>>>>> + * doesn't exist, or logging is enabled.
>>>>
>>>> First, Instead of "logging is enabled", it should be "force_pte is true",
>>>> since "force_pte" will be true when:
>>>>
>>>> 1) fault_supports_stage2_pmd_mappings() return false; or
>>>> 2) "logging is enabled" (e.g, in VM migration).
>>>>
>>>> Second, fallback some unsupported hugepage sizes (e.g, 64K hugepage with
>>>> 4K pages) to PTE is somewhat strange. And it will then _unexpectedly_
>>>> reach transparent_hugepage_adjust(), though no real adjustment will happen
>>>> since commit fd2ef358282c ("KVM: arm/arm64: Ensure only THP is candidate
>>>> for adjustment"). Keeping "vma_pagesize" there as it is will be better,
>>>> right?
>>>>
>>>> So I'd just simplify the logic like:
>>>
>>> We could fix this right in the beginning. See patch below:
>>>
>>>>
>>>> } else if (force_pte) {
>>>> vma_pagesize = PAGE_SIZE;
>>>> }
>>>>
>>>>
>>>> Will send a V2 later and waiting for your comments :)
>>>
>>>
>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>> index 30251e2..529331e 100644
>>> --- a/virt/kvm/arm/mmu.c
>>> +++ b/virt/kvm/arm/mmu.c
>>> @@ -1693,7 +1693,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>> return -EFAULT;
>>> }
>>>
>>> - vma_pagesize = vma_kernel_pagesize(vma);
>>> + /* If we are forced to map at page granularity, force the pagesize here */
>>> + vma_pagesize = force_pte ? PAGE_SIZE : vma_kernel_pagesize(vma);
>>> +
>>> /*
>>> * The stage2 has a minimum of 2 level table (For arm64 see
>>> * kvm_arm_setup_stage2()). Hence, we are guaranteed that we can
>>> @@ -1701,11 +1703,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>> * As for PUD huge maps, we must make sure that we have at least
>>> * 3 levels, i.e, PMD is not folded.
>>> */
>>> - if ((vma_pagesize == PMD_SIZE ||
>>> - (vma_pagesize == PUD_SIZE && kvm_stage2_has_pmd(kvm))) &&
>>> - !force_pte) {
>>> + if (vma_pagesize == PMD_SIZE ||
>>> + (vma_pagesize == PUD_SIZE && kvm_stage2_has_pmd(kvm)))
>>> gfn = (fault_ipa & huge_page_mask(hstate_vma(vma))) >> PAGE_SHIFT;
>>> - }
>>> +
>>> up_read(&current->mm->mmap_sem);
>>>
>>> /* We need minimum second+third level pages */
>
> A nicer implementation and easier to understand, thanks!
>
>> That's pretty interesting, because this is almost what we already have
>> in the NV code:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/tree/virt/kvm/arm/mmu.c?h=kvm-arm64/nv-wip-v5.0-rc7#n1752
>>
>> (note that force_pte is gone in that branch).
>
> haha :-) sorry about that. I haven't looked into the NV code yet, so ...
>
> But I'm still wondering: should we fix this wrong mapping size problem
> before NV is introduced? Since this problem has not much to do with NV,
> and 5.0 has already been released with this problem (and 5.1 will
> without fix ...).
>
> Just a personal idea, ignore it if unnecessary.

We definitely want to fix it now, and have the fix backported to older
versions. I can always rebase the NV branch on top of the current state
of mainline.

Thanks,

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