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

[Suzuki K Poulose]

Hi Zenghui,

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@xxxxxxxxxx> 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@xxxxxxx>
> > > > > Cc: Punit Agrawal <punit.agrawal@xxxxxxx>
> > > > > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > > > >
> > > > > ---
> > > > > 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 ...).

Yes, we must fix it. I will soon send out a patch copying on it.
Its just that I find some more issues around forcing the PTE
mappings with PUD huge pages. I will send something out soon.

Cheers
Suzuki