KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty

Attempt to allocate a new object instead of crashing KVM (and likely the
kernel) if a memory cache is unexpectedly empty.  Use GFP_ATOMIC for the
allocation as the caches are used while holding mmu_lock.  The immediate
BUG_ON() makes the code unnecessarily explosive and led to confusing
minimums being used in the past, e.g. allocating 4 objects where 1 would
suffice.
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200703023545.8771-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1061,6 +1061,15 @@ static void walk_shadow_page_lockless_end(struct kvm_vcpu *vcpu)
 	local_irq_enable();
 }
 
+static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
+					       gfp_t gfp_flags)
+{
+	if (mc->kmem_cache)
+		return kmem_cache_zalloc(mc->kmem_cache, gfp_flags);
+	else
+		return (void *)__get_free_page(gfp_flags);
+}
+
 static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
 {
 	void *obj;
@@ -1068,10 +1077,7 @@ static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min)
 	if (mc->nobjs >= min)
 		return 0;
 	while (mc->nobjs < ARRAY_SIZE(mc->objects)) {
-		if (mc->kmem_cache)
-			obj = kmem_cache_zalloc(mc->kmem_cache, GFP_KERNEL_ACCOUNT);
-		else
-			obj = (void *)__get_free_page(GFP_KERNEL_ACCOUNT);
+		obj = mmu_memory_cache_alloc_obj(mc, GFP_KERNEL_ACCOUNT);
 		if (!obj)
 			return mc->nobjs >= min ? 0 : -ENOMEM;
 		mc->objects[mc->nobjs++] = obj;
@@ -1119,8 +1125,11 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
 {
 	void *p;
 
-	BUG_ON(!mc->nobjs);
+	if (WARN_ON(!mc->nobjs))
+		p = mmu_memory_cache_alloc_obj(mc, GFP_ATOMIC | __GFP_ACCOUNT);
+	else
 		p = mc->objects[--mc->nobjs];
+	BUG_ON(!p);
 	return p;
 }