[PATCH RFC] KVM: x86/mmu: Remove KVM memory shrinker

[Vipin Sharma]

KVM memory shrinker is only used in the shadow paging. Most of the L1
guests are backed by TDP (Two Dimensional Paging) which do not use the
shrinker, only L2 guests are backed by shadow paging.

KVM memory shrinker can cause guests performance to degrade if any other
process (VM or non-VM) in the same or different cgroup in kernel causes
memory shrinker to run.

The KVM memory shrinker was introduced in 2008,
commit 3ee16c814511 ("KVM: MMU: allow the vm to shrink the kvm mmu
shadow caches"), each invocation of shrinker only released 1 shadow page
in 1 VM. This behavior was not effective until the batch zapping commit
was added in 2020, commit ebdb292dac79 ("KVM: x86/mmu: Batch zap MMU
pages when shrinking the slab"), which zaps multiple pages but still in
1 VM for each shrink invocation. Overall, this feature existed for many
years without providing meaningful benefit.

Signed-off-by: Vipin Sharma <vipinsh@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 99 ++----------------------------------------
 1 file changed, 3 insertions(+), 96 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 4e8d546431eb..80618c847ce2 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -178,7 +178,6 @@ 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;
 
 static void mmu_spte_set(u64 *sptep, u64 spte);
 
@@ -1658,16 +1657,9 @@ 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)
+static inline void kvm_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_mmu_free_page(struct kvm_mmu_page *sp)
@@ -1725,7 +1717,7 @@ static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct
 	 */
 	sp->mmu_valid_gen = vcpu->kvm->arch.mmu_valid_gen;
 	list_add(&sp->link, &vcpu->kvm->arch.active_mmu_pages);
-	kvm_mod_used_mmu_pages(vcpu->kvm, +1);
+	kvm_used_mmu_pages(vcpu->kvm, 1);
 	return sp;
 }
 
@@ -2341,7 +2333,7 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
 			list_add(&sp->link, invalid_list);
 		else
 			list_move(&sp->link, invalid_list);
-		kvm_mod_used_mmu_pages(kvm, -1);
+		kvm_used_mmu_pages(kvm, -1);
 	} else {
 		/*
 		 * Remove the active root from the active page list, the root
@@ -5801,11 +5793,6 @@ 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)
@@ -6159,77 +6146,6 @@ 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;
-	unsigned long freed = 0;
-
-	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--)
-			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;
-
-		idx = srcu_read_lock(&kvm->srcu);
-		write_lock(&kvm->mmu_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;
-	}
-
-	mutex_unlock(&kvm_lock);
-	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);
-}
-
-static struct shrinker mmu_shrinker = {
-	.count_objects = mmu_shrink_count,
-	.scan_objects = mmu_shrink_scan,
-	.seeks = DEFAULT_SEEKS * 10,
-};
-
 static void mmu_destroy_caches(void)
 {
 	kmem_cache_destroy(pte_list_desc_cache);
@@ -6325,13 +6241,6 @@ 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))
-		goto out;
-
-	ret = register_shrinker(&mmu_shrinker);
-	if (ret)
-		goto out;
-
 	return 0;
 
 out:
@@ -6350,8 +6259,6 @@ 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);
-	unregister_shrinker(&mmu_shrinker);
 }
 
 /*
-- 
2.36.0.464.gb9c8b46e94-goog