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    Subject[PATCH v3 05/21] KVM: x86/mmu: Try to avoid crashing KVM if a MMU memory cache is empty
    Date
    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>
    ---
    arch/x86/kvm/mmu/mmu.c | 21 +++++++++++++++------
    1 file changed, 15 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
    index 94b27adab6ba..748235877def 100644
    --- a/arch/x86/kvm/mmu/mmu.c
    +++ b/arch/x86/kvm/mmu/mmu.c
    @@ -1060,6 +1060,15 @@ 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;
    @@ -1067,10 +1076,7 @@ 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;
    @@ -1118,8 +1124,11 @@ 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;
    }

    --
    2.26.0
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