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

From: Mingwei Zhang
Date: Tue Jan 03 2023 - 14:32:52 EST

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In reply to: Vipin Sharma: "Re: [Patch v3 1/9] KVM: x86/mmu: Repurpose KVM MMU shrinker to purge shadow page caches"
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On Wed, Dec 21, 2022 at 6:35 PM Vipin Sharma <vipinsh@xxxxxxxxxx> 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.
>
> 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);
> +}

I think the mmu_cache allocation and deallocation may cause the usage
of GFP_ATOMIC (as observed by other reviewers as well). Adding a new
lock would definitely sound like a plan, but I think it might affect
the performance. Alternatively, I am wondering if we could use a
mmu_cache_sequence similar to mmu_notifier_seq to help avoid the
concurrency?

Similar to mmu_notifier_seq, mmu_cache_sequence should be protected by
mmu write lock. In the page fault path, each vcpu has to collect a
snapshot of mmu_cache_sequence before calling into
mmu_topup_memory_caches() and check the value again when holding the
mmu lock. If the value is different, that means the mmu_shrinker has
removed the cache objects and because of that, the vcpu should retry.

> +
> 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);
> 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) {
> + 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);
> + }
>
> - 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
>

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