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

From: Suzuki K Poulose
Date: Wed Apr 10 2019 - 04:39:08 EST

On 10/04/2019 03:20, Zenghui Yu wrote:

On 2019/4/9 22:59, Suzuki K Poulose wrote:
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
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

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

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
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 kvm_release_pfn_clean(pfn);


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;

+ÂÂÂÂ * If the memslot is _not_ backed by hugetlbfs, then check if it
+ÂÂÂÂ * can be backed by transparent hugepages.
+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

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.
-ÂÂÂÂÂÂÂ if (transparent_hugepage_adjust(&pfn, &fault_ipa))
+ÂÂÂÂÂÂÂ if (fault_supports_stage2_huge_mappings(memslot, hva,
+ÂÂÂÂÂÂÂÂÂÂÂ transparent_hugepage_adjust(&pfn, &fault_ipa))

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.

Hi Suzuki,

Yes, I agree, thanks.

Cool, I have a patch to fix this properly and two other patches to clean up
and unify the way we handle the THP backed hugepages. Will send them out after
a bit of testing, later today.