linux-kernel - Re: [PATCH v3] kvm: mmu: lazy collapse small sptes into large sptes

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Date:	Tue, 14 Apr 2015 13:25:39 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Andres Lagar-Cavilla <andreslc@...gle.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH v3] kvm: mmu: lazy collapse small sptes into large sptes

Hi Andres,
On Fri, Apr 10, 2015 at 11:05:26AM -0700, Andres Lagar-Cavilla wrote:
[...]
>> +               if (sp->role.direct &&
>> +                       !kvm_is_reserved_pfn(pfn) &&
>> +                       PageTransCompound(pfn_to_page(pfn))) {
>
>Not your fault, but PageTransCompound is very unhappy naming, as it
>also yields true for PageHuge. Suggestion: document this check covers
>static hugetlbfs, or switch to PageCompound() check.
>
>A slightly bolder approach would be to refactor and reuse the nearly
>identical check done in transparent_hugepage_adjust, instead of
>open-coding here. In essence this code is asking for the same check,
>plus the out-of-band check for static hugepages.

PageCompound() check still return true for both transparent huge pages
and hugetlbfs pages, !PageHuge(page) && PageTransHuge(page) check can 
guarantee to catch the right transparent huge pages just as my old commit 
e76d30e20be5fc ("mm/hwpoison: fix test for a transparent huge page"). 
I will send a patch to fix this.

>
>
>> +                       drop_spte(kvm, sptep);
>> +                       sptep = rmap_get_first(*rmapp, &iter);
>> +                       need_tlb_flush = 1;
>> +               } else
>> +                       sptep = rmap_get_next(&iter);
>> +       }
>> +
>> +       return need_tlb_flush;
>> +}
>> +
>> +void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>> +                       struct kvm_memory_slot *memslot)
>> +{
>> +       bool flush = false;
>> +       unsigned long *rmapp;
>> +       unsigned long last_index, index;
>> +       gfn_t gfn_start, gfn_end;
>> +
>> +       spin_lock(&kvm->mmu_lock);
>> +
>> +       gfn_start = memslot->base_gfn;
>> +       gfn_end = memslot->base_gfn + memslot->npages - 1;
>> +
>> +       if (gfn_start >= gfn_end)
>> +               goto out;
>
>I don't understand the value of this check here. Are we looking for a
>broken memslot? Shouldn't this be a BUG_ON? Is this the place to care
>about these things? npages is capped to KVM_MEM_MAX_NR_PAGES, i.e.
>2^31. A 64 bit overflow would be caused by a gigantic gfn_start which
>would be trouble in many other ways.
>
>All this to say: please remove the above 5 lines and make code simpler.

I will send a patch to cleanup it. Thanks for your review. :)

Regards,
Wanpeng Li

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