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

[Wanpeng Li <wanpeng.li@linux.intel.com>]

On Mon, Apr 13, 2015 at 09:45:39AM +0800, Xiao Guangrong 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@linux.intel.com> 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@linux.intel.com>
>>>Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>>
>>Hi, apologies for late review (vacation), but wanted to bring
>>attention to a few matters:
>
>No problem, your comments are really valuable to us. :)
>
>>
>>>
>>>---
>>>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().
>
>>
>>>+               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?
>
>Wanpeng, could you please post a patch to address Andres's comments?

Yeah, I will post a patch which is based on this one after merge window. :)

Regards,
Wanpeng Li