KVM: x86/mmu: Batch zap MMU pages when shrinking the slab

Use the recently introduced kvm_mmu_zap_oldest_mmu_pages() to batch zap
MMU pages when shrinking a slab.  This fixes a long standing issue where
KVM's shrinker implementation is completely ineffective due to zapping
only a single page.  E.g. without batch zapping, forcing a scan via
drop_caches basically has no impact on a VM with ~2k shadow pages.  With
batch zapping, the number of shadow pages can be reduced to a few
hundred pages in one or two runs of drop_caches.

Note, if the default batch size (currently 128) is problematic, e.g.
zapping 128 pages holds mmu_lock for too long, KVM can bound the batch
size by setting @batch in mmu_shrinker.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200623193542.7554-4-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 86abe2d..8083ec3 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2816,19 +2816,6 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
        }
 }
 
-static bool prepare_zap_oldest_mmu_page(struct kvm *kvm,
-                                       struct list_head *invalid_list)
-{
-       struct kvm_mmu_page *sp;
-
-       if (list_empty(&kvm->arch.active_mmu_pages))
-               return false;
-
-       sp = list_last_entry(&kvm->arch.active_mmu_pages,
-                            struct kvm_mmu_page, link);
-       return kvm_mmu_prepare_zap_page(kvm, sp, invalid_list);
-}
-
 static unsigned long kvm_mmu_zap_oldest_mmu_pages(struct kvm *kvm,
                                                  unsigned long nr_to_zap)
 {
@@ -6116,9 +6103,7 @@ mmu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
                        goto unlock;
                }
 
-               if (prepare_zap_oldest_mmu_page(kvm, &invalid_list))
-                       freed++;
-               kvm_mmu_commit_zap_page(kvm, &invalid_list);
+               freed = kvm_mmu_zap_oldest_mmu_pages(kvm, sc->nr_to_scan);
 
 unlock:
                spin_unlock(&kvm->mmu_lock);