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

[Andres Lagar-Cavilla]

On Sun, Apr 12, 2015 at 6:45 PM, Xiao Guangrong
<guangrong.xiao@xxxxxxxxxxxxxxx> wrote:
>
>
> On 04/11/2015 02:05 AM, Andres Lagar-Cavilla wrote:
>>
>> On Fri, Apr 3, 2015 at 12:40 AM, Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>> wrote:
>>>
>>>
>>> There are two scenarios for the requirement of collapsing small sptes
>>> into large sptes.
>>> - dirty logging tracks sptes in 4k granularity, so large sptes are split,
>>>    the large sptes will be reallocated in the destination machine and the
>>>    guest in the source machine will be destroyed when live migration
>>> successfully.
>>>    However, the guest in the source machine will continue to run if live
>>> migration
>>>    fail due to some reasons, the sptes still keep small which lead to bad
>>>    performance.
>>> - our customers write tools to track the dirty speed of guests by EPT D
>>> bit/PML
>>>    in order to determine the most appropriate one to be live migrated,
>>> however
>>>    sptes will still keep small after tracking dirty speed.
>>>
>>> This patch introduce lazy collapse small sptes into large sptes, the
>>> memory region
>>> will be scanned on the ioctl context when dirty log is stopped, the ones
>>> which can
>>> be collapsed into large pages will be dropped during the scan, it depends
>>> the on
>>> later #PF to reallocate all large sptes.
>>>
>>> Reviewed-by: Xiao Guangrong <guangrong.xiao@xxxxxxxxxxxxxxx>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxxxxxx>
>>
>>
>> Hi, apologies for late review (vacation), but wanted to bring
>> attention to a few matters:
>
>
> No problem, your comments are really valuable to us. :)

Glad to know, thanks.

>
>
>>
>>>
>>> ---
>>> v2 -> v3:
>>>   * update comments
>>>   * fix infinite for loop
>>> v1 -> v2:
>>>   * use 'bool' instead of 'int'
>>>   * add more comments
>>>   * fix can not get the next spte after drop the current spte
>>>
>>>   arch/x86/include/asm/kvm_host.h |  2 ++
>>>   arch/x86/kvm/mmu.c              | 73
>>> +++++++++++++++++++++++++++++++++++++++++
>>>   arch/x86/kvm/x86.c              | 19 +++++++++++
>>>   3 files changed, 94 insertions(+)
>>>
>>> diff --git a/arch/x86/include/asm/kvm_host.h
>>> b/arch/x86/include/asm/kvm_host.h
>>> index 30b28dc..91b5bdb 100644
>>> --- a/arch/x86/include/asm/kvm_host.h
>>> +++ b/arch/x86/include/asm/kvm_host.h
>>> @@ -854,6 +854,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64
>>> accessed_mask,
>>>   void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
>>>   void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
>>>                                        struct kvm_memory_slot *memslot);
>>> +void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>>> +                                       struct kvm_memory_slot *memslot);
>>>   void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
>>>                                     struct kvm_memory_slot *memslot);
>>>   void kvm_mmu_slot_largepage_remove_write_access(struct kvm *kvm,
>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>> index cee7592..ba002a0 100644
>>> --- a/arch/x86/kvm/mmu.c
>>> +++ b/arch/x86/kvm/mmu.c
>>> @@ -4465,6 +4465,79 @@ void kvm_mmu_slot_remove_write_access(struct kvm
>>> *kvm,
>>>                  kvm_flush_remote_tlbs(kvm);
>>>   }
>>>
>>> +static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
>>> +               unsigned long *rmapp)
>>> +{
>>> +       u64 *sptep;
>>> +       struct rmap_iterator iter;
>>> +       int need_tlb_flush = 0;
>>> +       pfn_t pfn;
>>> +       struct kvm_mmu_page *sp;
>>> +
>>> +       for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
>>> +               BUG_ON(!(*sptep & PT_PRESENT_MASK));
>>> +
>>> +               sp = page_header(__pa(sptep));
>>> +               pfn = spte_to_pfn(*sptep);
>>> +
>>> +               /*
>>> +                * Lets support EPT only for now, there still needs to
>>> figure
>>> +                * out an efficient way to let these codes be aware what
>>> mapping
>>> +                * level used in guest.
>>> +                */
>>> +               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.
>
>
> I agree.
>
>
>>
>>
>>> +                       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.
>
>
> Yes, this check is unnecessary indeed.
>
>>
>>> +
>>> +       rmapp = memslot->arch.rmap[0];
>>> +       last_index = gfn_to_index(gfn_end, memslot->base_gfn,
>>> +                                       PT_PAGE_TABLE_LEVEL);
>>> +
>>> +       for (index = 0; index <= last_index; ++index, ++rmapp) {
>>
>>
>> One could argue that the cleaner iteration should be over the gfn
>> space covered by the memslot, thus leaving the gfn <--> rmap <--> spte
>> interactions hidden under the hood of __gfn_to_rmap. That yields much
>> cleaner (IMHO) code:
>>
>>      for (gfn = memslot->base_gfn; gfn <= memslot->base_gfn +
>> memslot->npages; gfn++) {
>>          flush |= kvm_mmu_zap_collapsible_spte(kvm, __gfn_to_rmap(gfn,
>> 1, memslot));
>>          ....
>>
>> Now you can also get rid of index, last_index and rmapp. And more
>> importantly, the code is more understandable, and follows pattern as
>> established in x86/kvm/mmu.
>
>
> Do not have strong opinion on it. Current code also has this style, please
> refer to kvm_mmu_slot_remove_write_access().
>

Ugh. You're right. Not your fight to pick, I guess.

>>
>>> +               if (*rmapp)
>>> +                       flush |= kvm_mmu_zap_collapsible_spte(kvm,
>>> rmapp);
>>> +
>>> +               if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
>>> +                       if (flush) {
>>> +                               kvm_flush_remote_tlbs(kvm);
>>> +                               flush = false;
>>> +                       }
>>> +                       cond_resched_lock(&kvm->mmu_lock);
>>
>>
>> Relinquishing this spinlock is problematic, because
>> commit_memory_region has not gotten around to removing write
>> protection. Are you certain no new write-protected PTEs will be
>> inserted by a racing fault that sneaks in while the spinlock is
>> relinquished?
>>
>
> I do not know clearly about the problem, new spte creation will be based on
> the host mapping, i.e, the huge mapping on shadow page table will be
> sync-ed with huge mapping on host. Could you please detail the problem?
>

It seems that, prior to calling commit_memory_region, the new memslot
with dirty_bitmap = NULL is made effective. This will ensure
force_pt_level is false in tdp_page_fault, and huge mappings are
observed. So all is well and I was fud'ing. N.B.: software shadow
mode?

> Wanpeng, could you please post a patch to address Andres's comments?
>
Awesome, thanks
Andres


-- 
Andres Lagar-Cavilla | Google Kernel Team | andreslc@xxxxxxxxxx
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