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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>
This commit is contained in:
Sean Christopherson 2020-07-02 19:35:29 -07:00 committed by Paolo Bonzini
parent 284aa86868
commit 53a3f48771
1 changed files with 15 additions and 6 deletions
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21
arch/x86/kvm/mmu/mmu.c
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@ -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);
p = mc->objects[--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;
}