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

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Date:   Mon, 8 Apr 2019 19:40:09 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     yuzenghui@...wei.com, linux-arm-kernel@...ts.infradead.org
Cc:     kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, eric.auger@...hat.com,
        marc.zyngier@....com, christoffer.dall@....com,
        zhengxiang9@...wei.com, andrew.murray@....com,
        wanghaibin.wang@...wei.com
Subject: Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa
 backed by THP

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.
> 
>>>> +        /* Skip memslots with unaligned IPA and user address */
>>>> +        if ((gfn & mask) != (pfn & mask))
>>>> +            return false;
>>>>           if (pfn & mask) {
>>>>               *ipap &= PMD_MASK;
>>>>               kvm_release_pfn_clean(pfn);
>>>>
>>>
>>> ---8>---
>>>
>>> Rework fault_supports_stage2_huge_mapping(), let it check THP again.
>>>
>>> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
>>> ---
>>>   virt/kvm/arm/mmu.c | 11 ++++++++++-
>>>   1 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,
>>>       uaddr_end = uaddr_start + size;
>>>
>>>       /*
>>> +     * If the memslot is _not_ backed by hugetlbfs, then check if it
>>> +     * can be backed by transparent hugepages.
>>> +     *
>>> +     * Currently only PMD_SIZE THPs are supported, revisit it later.
>>> +     */
>>> +    if (map_size == PAGE_SIZE)
>>> +        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,
  		 * currently supported. This code will need to be
  		 * updated to support other THP sizes.
  		 */
-		if (transparent_hugepage_adjust(&pfn, &fault_ipa))
+		if (fault_supports_stage2_huge_mappings(memslot, hva, PMD_SIZE) &&
+		    transparent_hugepage_adjust(&pfn, &fault_ipa))
  			vma_pagesize = PMD_SIZE;
  	}

-- 
2.7.4



Suzuki