Re: [Patch v3 1/9] KVM: x86/mmu: Repurpose KVM MMU shrinker to purge shadow page caches

[Vipin Sharma]

On Thu, Dec 29, 2022 at 1:55 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> On Wed, Dec 21, 2022 at 06:34:49PM -0800, Vipin Sharma wrote:
> > mmu_shrink_scan() is very disruptive to VMs. It picks the first
> > VM in the vm_list, zaps the oldest page which is most likely an upper
> > level SPTEs and most like to be reused. Prior to TDP MMU, this is even
> > more disruptive in nested VMs case, considering L1 SPTEs will be the
> > oldest even though most of the entries are for L2 SPTEs.
> >
> > As discussed in
> > https://lore.kernel.org/lkml/Y45dldZnI6OIf+a5@xxxxxxxxxx/
> > shrinker logic has not be very useful in actually keeping VMs performant
> > and reducing memory usage.
> >
> > Change mmu_shrink_scan() to free pages from the vCPU's shadow page
> > cache.  Freeing pages from cache doesn't cause vCPU exits, therefore, a
> > VM's performance should not be affected.
>
> Can you split this commit up? e.g. First drop the old shrinking logic in
> one commit (but leave the shrinking infrastructure in place). Then a
> commit to make the shrinker free the per-vCPU shadow page caches. And
> then perhaps another to make the shrinker free the per-VM shadow page
> cache used for eager splitting.
>

Sounds good, I will separate it in two parts, one for dropping old
logic, one for adding per vcpu shadow page caches. Patch 3 is enabling
shrinkerto free per-VM shadow page.

> >
> > This also allows to change cache capacities without worrying too much
> > about high memory usage in cache.
> >
> > Tested this change by running dirty_log_perf_test while dropping cache
> > via "echo 2 > /proc/sys/vm/drop_caches" at 1 second interval
> > continuously. There were WARN_ON(!mc->nobjs) messages printed in kernel
> > logs from kvm_mmu_memory_cache_alloc(), which is expected.
> >
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Vipin Sharma <vipinsh@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h |   5 +
> >  arch/x86/kvm/mmu/mmu.c          | 163 +++++++++++++++++++-------------
> >  arch/x86/kvm/mmu/mmu_internal.h |   2 +
> >  arch/x86/kvm/mmu/tdp_mmu.c      |   3 +-
> >  include/linux/kvm_host.h        |   1 +
> >  virt/kvm/kvm_main.c             |  11 ++-
> >  6 files changed, 114 insertions(+), 71 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index aa4eb8cfcd7e..89cc809e4a00 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -786,6 +786,11 @@ struct kvm_vcpu_arch {
> >       struct kvm_mmu_memory_cache mmu_shadowed_info_cache;
> >       struct kvm_mmu_memory_cache mmu_page_header_cache;
> >
> > +     /*
> > +      * Protects change in size of mmu_shadow_page_cache cache.
> > +      */
> > +     spinlock_t mmu_shadow_page_cache_lock;
> > +
> >       /*
> >        * QEMU userspace and the guest each have their own FPU state.
> >        * In vcpu_run, we switch between the user and guest FPU contexts.
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index 254bc46234e0..157417e1cb6e 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -164,7 +164,10 @@ struct kvm_shadow_walk_iterator {
> >
> >  static struct kmem_cache *pte_list_desc_cache;
> >  struct kmem_cache *mmu_page_header_cache;
> > -static struct percpu_counter kvm_total_used_mmu_pages;
> > +/*
> > + * Total number of unused pages in MMU shadow page cache.
> > + */
> > +static struct percpu_counter kvm_total_unused_mmu_pages;
> >
> >  static void mmu_spte_set(u64 *sptep, u64 spte);
> >
> > @@ -655,6 +658,22 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
> >       }
> >  }
> >
> > +static int mmu_topup_sp_memory_cache(struct kvm_mmu_memory_cache *cache,
> > +                                  spinlock_t *cache_lock)
> > +{
> > +     int orig_nobjs;
> > +     int r;
> > +
> > +     spin_lock(cache_lock);
> > +     orig_nobjs = cache->nobjs;
> > +     r = kvm_mmu_topup_memory_cache(cache, PT64_ROOT_MAX_LEVEL);
> > +     if (orig_nobjs != cache->nobjs)
> > +             percpu_counter_add(&kvm_total_unused_mmu_pages,
> > +                                (cache->nobjs - orig_nobjs));
> > +     spin_unlock(cache_lock);
> > +     return r;
> > +}
> > +
> >  static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> >  {
> >       int r;
> > @@ -664,8 +683,8 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> >                                      1 + PT64_ROOT_MAX_LEVEL + PTE_PREFETCH_NUM);
> >       if (r)
> >               return r;
> > -     r = kvm_mmu_topup_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> > -                                    PT64_ROOT_MAX_LEVEL);
> > +     r = mmu_topup_sp_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> > +                                   &vcpu->arch.mmu_shadow_page_cache_lock);
> >       if (r)
> >               return r;
> >       if (maybe_indirect) {
> > @@ -678,10 +697,25 @@ static int mmu_topup_memory_caches(struct kvm_vcpu *vcpu, bool maybe_indirect)
> >                                         PT64_ROOT_MAX_LEVEL);
> >  }
> >
> > +static void mmu_free_sp_memory_cache(struct kvm_mmu_memory_cache *cache,
> > +                                  spinlock_t *cache_lock)
> > +{
> > +     int orig_nobjs;
> > +
> > +     spin_lock(cache_lock);
> > +     orig_nobjs = cache->nobjs;
> > +     kvm_mmu_free_memory_cache(cache);
> > +     if (orig_nobjs)
> > +             percpu_counter_sub(&kvm_total_unused_mmu_pages, orig_nobjs);
> > +
> > +     spin_unlock(cache_lock);
> > +}
>
> It would be nice to avoid adding these wrapper functions.
>
> Once you add a mutex to protect the caches from being freed while vCPUs
> are in the middle of a page fault you can drop the spin lock. After that
> the only reason to have these wrappers is to update
> kvm_total_unused_mmu_pages.
>
> Do we really need kvm_total_unused_mmu_pages? Why not just dynamically
> calculate the number of of unused pages in mmu_shrink_count()? Or just
> estimate the count, e.g. num_vcpus * KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE?
> Or have per-VM or per-vCPU shrinkers to avoid needing to do any
> aggregation?
>

I think we can drop this, by default we can return num_kvms *
num_vcpus * nodes * KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE

Whenever mmu_shrink_scan() is called if there are no pages to free
then return SHRINK_STOP which will stop any subsequent calls during
that time.


> > +
> >  static void mmu_free_memory_caches(struct kvm_vcpu *vcpu)
> >  {
> >       kvm_mmu_free_memory_cache(&vcpu->arch.mmu_pte_list_desc_cache);
> > -     kvm_mmu_free_memory_cache(&vcpu->arch.mmu_shadow_page_cache);
> > +     mmu_free_sp_memory_cache(&vcpu->arch.mmu_shadow_page_cache,
> > +                              &vcpu->arch.mmu_shadow_page_cache_lock);
> >       kvm_mmu_free_memory_cache(&vcpu->arch.mmu_shadowed_info_cache);
>
> mmu_shadowed_info_cache can be freed by the shrinker as well.
>
> >       kvm_mmu_free_memory_cache(&vcpu->arch.mmu_page_header_cache);
> >  }
> > @@ -1693,27 +1727,15 @@ static int is_empty_shadow_page(u64 *spt)
> >  }
> >  #endif
> >
> > -/*
> > - * This value is the sum of all of the kvm instances's
> > - * kvm->arch.n_used_mmu_pages values.  We need a global,
> > - * aggregate version in order to make the slab shrinker
> > - * faster
> > - */
> > -static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, long nr)
> > -{
> > -     kvm->arch.n_used_mmu_pages += nr;
> > -     percpu_counter_add(&kvm_total_used_mmu_pages, nr);
> > -}
> > -
> >  static void kvm_account_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> >  {
> > -     kvm_mod_used_mmu_pages(kvm, +1);
> > +     kvm->arch.n_used_mmu_pages++;
> >       kvm_account_pgtable_pages((void *)sp->spt, +1);
> >  }
> >
> >  static void kvm_unaccount_mmu_page(struct kvm *kvm, struct kvm_mmu_page *sp)
> >  {
> > -     kvm_mod_used_mmu_pages(kvm, -1);
> > +     kvm->arch.n_used_mmu_pages--;
> >       kvm_account_pgtable_pages((void *)sp->spt, -1);
> >  }
> >
> > @@ -2150,8 +2172,31 @@ struct shadow_page_caches {
> >       struct kvm_mmu_memory_cache *page_header_cache;
> >       struct kvm_mmu_memory_cache *shadow_page_cache;
> >       struct kvm_mmu_memory_cache *shadowed_info_cache;
> > +     /*
> > +      * Protects change in size of shadow_page_cache cache.
> > +      */
> > +     spinlock_t *shadow_page_cache_lock;
> >  };
> >
> > +void *kvm_mmu_sp_memory_cache_alloc(struct kvm_mmu_memory_cache *shadow_page_cache,
> > +                                 spinlock_t *cache_lock)
> > +{
> > +     int orig_nobjs;
> > +     void *page;
> > +
> > +     if (!cache_lock) {
> > +             spin_lock(cache_lock);
> > +             orig_nobjs = shadow_page_cache->nobjs;
> > +     }
> > +     page = kvm_mmu_memory_cache_alloc(shadow_page_cache);
> > +     if (!cache_lock) {
> > +             if (orig_nobjs)
> > +                     percpu_counter_dec(&kvm_total_unused_mmu_pages);
> > +             spin_unlock(cache_lock);
> > +     }
> > +     return page;
> > +}
> > +
> >  static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
> >                                                     struct shadow_page_caches *caches,
> >                                                     gfn_t gfn,
> > @@ -2161,7 +2206,8 @@ static struct kvm_mmu_page *kvm_mmu_alloc_shadow_page(struct kvm *kvm,
> >       struct kvm_mmu_page *sp;
> >
> >       sp = kvm_mmu_memory_cache_alloc(caches->page_header_cache);
> > -     sp->spt = kvm_mmu_memory_cache_alloc(caches->shadow_page_cache);
> > +     sp->spt = kvm_mmu_sp_memory_cache_alloc(caches->shadow_page_cache,
> > +                                             caches->shadow_page_cache_lock);
> >       if (!role.direct)
> >               sp->shadowed_translation = kvm_mmu_memory_cache_alloc(caches->shadowed_info_cache);
> >
> > @@ -2218,6 +2264,7 @@ static struct kvm_mmu_page *kvm_mmu_get_shadow_page(struct kvm_vcpu *vcpu,
> >               .page_header_cache = &vcpu->arch.mmu_page_header_cache,
> >               .shadow_page_cache = &vcpu->arch.mmu_shadow_page_cache,
> >               .shadowed_info_cache = &vcpu->arch.mmu_shadowed_info_cache,
> > +             .shadow_page_cache_lock = &vcpu->arch.mmu_shadow_page_cache_lock
> >       };
> >
> >       return __kvm_mmu_get_shadow_page(vcpu->kvm, vcpu, &caches, gfn, role);
> > @@ -5916,6 +5963,7 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
> >       vcpu->arch.mmu_page_header_cache.gfp_zero = __GFP_ZERO;
> >
> >       vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
> > +     spin_lock_init(&vcpu->arch.mmu_shadow_page_cache_lock);
> >
> >       vcpu->arch.mmu = &vcpu->arch.root_mmu;
> >       vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
> > @@ -6051,11 +6099,6 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
> >               kvm_tdp_mmu_zap_invalidated_roots(kvm);
> >  }
> >
> > -static bool kvm_has_zapped_obsolete_pages(struct kvm *kvm)
> > -{
> > -     return unlikely(!list_empty_careful(&kvm->arch.zapped_obsolete_pages));
> > -}
> > -
> >  static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,
> >                       struct kvm_memory_slot *slot,
> >                       struct kvm_page_track_notifier_node *node)
> > @@ -6277,6 +6320,7 @@ static struct kvm_mmu_page *shadow_mmu_get_sp_for_split(struct kvm *kvm, u64 *hu
> >       /* Direct SPs do not require a shadowed_info_cache. */
> >       caches.page_header_cache = &kvm->arch.split_page_header_cache;
> >       caches.shadow_page_cache = &kvm->arch.split_shadow_page_cache;
> > +     caches.shadow_page_cache_lock = NULL;
> >
> >       /* Safe to pass NULL for vCPU since requesting a direct SP. */
> >       return __kvm_mmu_get_shadow_page(kvm, NULL, &caches, gfn, role);
> > @@ -6646,66 +6690,49 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen)
> >  static unsigned long
> >  mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> >  {
> > -     struct kvm *kvm;
> > -     int nr_to_scan = sc->nr_to_scan;
> > +     struct kvm_mmu_memory_cache *cache;
> > +     struct kvm *kvm, *first_kvm = NULL;
> >       unsigned long freed = 0;
> > +     /* spinlock for memory cache */
> > +     spinlock_t *cache_lock;
> > +     struct kvm_vcpu *vcpu;
> > +     unsigned long i;
> >
> >       mutex_lock(&kvm_lock);
> >
> >       list_for_each_entry(kvm, &vm_list, vm_list) {
> > -             int idx;
> > -             LIST_HEAD(invalid_list);
> > -
> > -             /*
> > -              * Never scan more than sc->nr_to_scan VM instances.
> > -              * Will not hit this condition practically since we do not try
> > -              * to shrink more than one VM and it is very unlikely to see
> > -              * !n_used_mmu_pages so many times.
> > -              */
> > -             if (!nr_to_scan--)
> > +             if (first_kvm == kvm)
> >                       break;
> > -             /*
> > -              * n_used_mmu_pages is accessed without holding kvm->mmu_lock
> > -              * here. We may skip a VM instance errorneosly, but we do not
> > -              * want to shrink a VM that only started to populate its MMU
> > -              * anyway.
> > -              */
> > -             if (!kvm->arch.n_used_mmu_pages &&
> > -                 !kvm_has_zapped_obsolete_pages(kvm))
> > -                     continue;
> > +             if (!first_kvm)
> > +                     first_kvm = kvm;
> > +             list_move_tail(&kvm->vm_list, &vm_list);
> >
> > -             idx = srcu_read_lock(&kvm->srcu);
> > -             write_lock(&kvm->mmu_lock);
> > +             kvm_for_each_vcpu(i, vcpu, kvm) {
>
> What protects this from racing with vCPU creation/deletion?
>
> > +                     cache = &vcpu->arch.mmu_shadow_page_cache;
> > +                     cache_lock = &vcpu->arch.mmu_shadow_page_cache_lock;
> > +                     if (READ_ONCE(cache->nobjs)) {
> > +                             spin_lock(cache_lock);
> > +                             freed += kvm_mmu_empty_memory_cache(cache);
> > +                             spin_unlock(cache_lock);
> > +                     }
>
> What about freeing kvm->arch.split_shadow_page_cache as well?
>
> >
> > -             if (kvm_has_zapped_obsolete_pages(kvm)) {
> > -                     kvm_mmu_commit_zap_page(kvm,
> > -                           &kvm->arch.zapped_obsolete_pages);
> > -                     goto unlock;
> >               }
> >
> > -             freed = kvm_mmu_zap_oldest_mmu_pages(kvm, sc->nr_to_scan);
> > -
> > -unlock:
> > -             write_unlock(&kvm->mmu_lock);
> > -             srcu_read_unlock(&kvm->srcu, idx);
> > -
> > -             /*
> > -              * unfair on small ones
> > -              * per-vm shrinkers cry out
> > -              * sadness comes quickly
> > -              */
> > -             list_move_tail(&kvm->vm_list, &vm_list);
> > -             break;
> > +             if (freed >= sc->nr_to_scan)
> > +                     break;
> >       }
> >
> > +     if (freed)
> > +             percpu_counter_sub(&kvm_total_unused_mmu_pages, freed);
> >       mutex_unlock(&kvm_lock);
> > +     percpu_counter_sync(&kvm_total_unused_mmu_pages);
> >       return freed;
> >  }
> >
> >  static unsigned long
> >  mmu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
> >  {
> > -     return percpu_counter_read_positive(&kvm_total_used_mmu_pages);
> > +     return percpu_counter_sum_positive(&kvm_total_unused_mmu_pages);
> >  }
> >
> >  static struct shrinker mmu_shrinker = {
> > @@ -6820,7 +6847,7 @@ int kvm_mmu_vendor_module_init(void)
> >       if (!mmu_page_header_cache)
> >               goto out;
> >
> > -     if (percpu_counter_init(&kvm_total_used_mmu_pages, 0, GFP_KERNEL))
> > +     if (percpu_counter_init(&kvm_total_unused_mmu_pages, 0, GFP_KERNEL))
> >               goto out;
> >
> >       ret = register_shrinker(&mmu_shrinker, "x86-mmu");
> > @@ -6830,7 +6857,7 @@ int kvm_mmu_vendor_module_init(void)
> >       return 0;
> >
> >  out_shrinker:
> > -     percpu_counter_destroy(&kvm_total_used_mmu_pages);
> > +     percpu_counter_destroy(&kvm_total_unused_mmu_pages);
> >  out:
> >       mmu_destroy_caches();
> >       return ret;
> > @@ -6847,7 +6874,7 @@ void kvm_mmu_destroy(struct kvm_vcpu *vcpu)
> >  void kvm_mmu_vendor_module_exit(void)
> >  {
> >       mmu_destroy_caches();
> > -     percpu_counter_destroy(&kvm_total_used_mmu_pages);
> > +     percpu_counter_destroy(&kvm_total_unused_mmu_pages);
> >       unregister_shrinker(&mmu_shrinker);
> >  }
> >
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index ac00bfbf32f6..c2a342028b6a 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -325,4 +325,6 @@ void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> >  void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> >  void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> >
> > +void *kvm_mmu_sp_memory_cache_alloc(struct kvm_mmu_memory_cache *shadow_page_cache,
> > +                                 spinlock_t *cache_lock);
> >  #endif /* __KVM_X86_MMU_INTERNAL_H */
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 764f7c87286f..4974fa96deff 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -264,7 +264,8 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp(struct kvm_vcpu *vcpu)
> >       struct kvm_mmu_page *sp;
> >
> >       sp = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_page_header_cache);
> > -     sp->spt = kvm_mmu_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache);
> > +     sp->spt = kvm_mmu_sp_memory_cache_alloc(&vcpu->arch.mmu_shadow_page_cache,
> > +                                             &vcpu->arch.mmu_shadow_page_cache_lock);
> >
> >       return sp;
> >  }
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 01aad8b74162..efd9b38ea9a2 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1362,6 +1362,7 @@ void kvm_flush_remote_tlbs(struct kvm *kvm);
> >  int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min);
> >  int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min);
> >  int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc);
> > +int kvm_mmu_empty_memory_cache(struct kvm_mmu_memory_cache *mc);
> >  void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
> >  void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> >  #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 13e88297f999..f2d762878b97 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -438,8 +438,10 @@ int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc)
> >       return mc->nobjs;
> >  }
> >
> > -void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> > +int kvm_mmu_empty_memory_cache(struct kvm_mmu_memory_cache *mc)
> >  {
> > +     int freed = mc->nobjs;
> > +
> >       while (mc->nobjs) {
> >               if (mc->kmem_cache)
> >                       kmem_cache_free(mc->kmem_cache, mc->objects[--mc->nobjs]);
> > @@ -447,8 +449,13 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> >                       free_page((unsigned long)mc->objects[--mc->nobjs]);
> >       }
> >
> > -     kvfree(mc->objects);
> > +     return freed;
> > +}
> >
> > +void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> > +{
> > +     kvm_mmu_empty_memory_cache(mc);
> > +     kvfree(mc->objects);
> >       mc->objects = NULL;
> >       mc->capacity = 0;
> >  }
> > --
> > 2.39.0.314.g84b9a713c41-goog
> >