Messages in this thread | | | Subject | Re: [PATCH] kvm: arm: Skip stage2 huge mappings for unaligned ipa backed by THP | From | Suzuki K Poulose <> | Date | Tue, 9 Apr 2019 15:59:16 +0100 |
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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. >>> >>>>>> + /* 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@huawei.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.)
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. > >
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