Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa backed by THP

[Suzuki K Poulose]

Hi Zenghui

On 04/09/2019 09:05 AM, Zenghui Yu wrote:
> On 2019/4/9 2:40, Suzuki K Poulose wrote:
> > Hi Zenhui,
> >
> > On 04/08/2019 04:11 PM, Zenghui Yu wrote:
> > > Hi Suzuki,
> > >
> > > Thanks for the reply.
> >
> > ...
> >
> > > > > Hi Suzuki,
> > > > >
> > > > > Why not making use of fault_supports_stage2_huge_mapping()? Let it do
> > > > > some checks for us.
> > > > >
> > > > > fault_supports_stage2_huge_mapping() was intended to do a *two-step*
> > > > > check to tell us that can we create stage2 huge block mappings, and 
> > > > > this
> > > > > check is both for hugetlbfs and THP. With commit a80868f398554842b14,
> > > > > we pass PAGE_SIZE as "map_size" for normal size pages (which turned 
> > > > > out
> > > > > to be almost meaningless), and unfortunately the THP check no longer
> > > > > works.
> > > >
> > > > Thats correct.
> > > >
> > > > > So we want to rework *THP* check process. Your patch fixes the first
> > > > > checking-step, but the second is still missed, am I wrong?
> > > >
> > > > It fixes the step explicitly for the THP by making sure that the GPA 
> > > > and
> > > > the HVA are aligned to the map size.
> > >
> > > Yes, I understand how your patch had fixed the issue. But what I'm
> > > really concerned about here is the *second* checking-step in
> > > fault_supports_stage2_huge_mapping().
> > >
> > > We have to check if we are mapping a non-block aligned or non-block
> > > sized memslot, if so, we can not create block mappings for the beginning
> > > and end of this memslot. This is what the second part of
> > > fault_supports_stage2_huge_mapping() had done.
> > >
> > > I haven't seen this checking-step in your patch, did I miss something?
> >
> > I see.
> >
> > > > I don't think this calls for a VM_BUG_ON(). It is simply a case where
> > > > the GPA is not aligned to HVA, but for normal VMA that could be made 
> > > > THP.
> > > >
> > > > We had this VM_BUG_ON(), which would have never hit because we would
> > > > have set force_pte if they were not aligned.
> > >
> > > Yes, I agree.
> > >
> > > > > > +ÂÂÂÂÂÂÂ /* Skip memslots with unaligned IPA and user address */
> > > > > > +ÂÂÂÂÂÂÂ if ((gfn & mask) != (pfn & mask))
> > > > > > +ÂÂÂÂÂÂÂÂÂÂÂ return false;
> > > > > > ÂÂÂÂÂÂÂÂÂ if (pfn & mask) {
> > > > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ *ipap &= PMD_MASK;
> > > > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂ kvm_release_pfn_clean(pfn);
> > > > >
> > > > > ---8>---
> > > > >
> > > > > Rework fault_supports_stage2_huge_mapping(), let it check THP again.
> > > > >
> > > > > Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
> > > > > ---
> > > > > ÂÂvirt/kvm/arm/mmu.c | 11 ++++++++++-
> > > > > ÂÂ1 file changed, 10 insertions(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> > > > > index 27c9583..5e1b258 100644
> > > > > --- a/virt/kvm/arm/mmu.c
> > > > > +++ b/virt/kvm/arm/mmu.c
> > > > > @@ -1632,6 +1632,15 @@ static bool 
> > > > > fault_supports_stage2_huge_mapping(struct kvm_memory_slot *memslot,
> > > > > ÂÂÂÂÂ uaddr_end = uaddr_start + size;
> > > > >
> > > > > ÂÂÂÂÂ /*
> > > > > +ÂÂÂÂ * If the memslot is _not_ backed by hugetlbfs, then check if it
> > > > > +ÂÂÂÂ * can be backed by transparent hugepages.
> > > > > +ÂÂÂÂ *
> > > > > +ÂÂÂÂ * Currently only PMD_SIZE THPs are supported, revisit it later.
> > > > > +ÂÂÂÂ */
> > > > > +ÂÂÂ if (map_size == PAGE_SIZE)
> > > > > +ÂÂÂÂÂÂÂ map_size = PMD_SIZE;
> > > > > +
> > > >
> > > > This looks hackish. What is we support PUD_SIZE huge page in the future
> > > > ?
> > >
> > > Yes, this might make the code a little difficult to understand. But by
> > > doing so, we follow the same logic before commit a80868f398554842b14,
> > > that said, we do the two-step checking for normal size pages in
> > > fault_supports_stage2_huge_mapping(), to decide if we can create THP
> > > mappings for these pages.
> > >
> > > As for PUD_SIZE THPs, to be honest, I have no idea now :(
> >
> > How about the following diff ?
> >
> > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> > index 97b5417..98e5cec 100644
> > --- a/virt/kvm/arm/mmu.c
> > +++ b/virt/kvm/arm/mmu.c
> > @@ -1791,7 +1791,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
> > phys_addr_t fault_ipa,
> > ÂÂÂÂÂÂÂÂÂÂ * currently supported. This code will need to be
> > ÂÂÂÂÂÂÂÂÂÂ * updated to support other THP sizes.
> > ÂÂÂÂÂÂÂÂÂÂ */
> > -ÂÂÂÂÂÂÂ if (transparent_hugepage_adjust(&pfn, &fault_ipa))
> > +ÂÂÂÂÂÂÂ if (fault_supports_stage2_huge_mappings(memslot, hva, 
> > PMD_SIZE) &&
> > +ÂÂÂÂÂÂÂÂÂÂÂ transparent_hugepage_adjust(&pfn, &fault_ipa))
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂ vma_pagesize = PMD_SIZE;
> > ÂÂÂÂÂ }
>
> I think this is good enough for the issue.
>
> (One minor concern: With this change, it seems that we no longer need
> "force_pte" and can just use "logging_active" instead. But this is not
> much related to what we're fixing.)

I would still leave the force_pte there to avoid checking for a THP case
in a situation where we forced to PTE level mapping on a hugepage backed
VMA. It would serve to avoid another check.

Cheers
Suzuki


> thanks.