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

Suzuki K Poulose
2019-03-04T17:13:39Z

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 */

---

Suzuki