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

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Date:   Tue, 9 Apr 2019 16:05:58 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Suzuki K Poulose <suzuki.poulose@....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



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.
>>
>>>>> +        /* 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;
>       }

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.)


thanks.