Re: [PATCH 2/2] KVM: x86/mmu: Use MMU shrinker to shrink KVM MMU memory caches

[David Matlack]

On 2024-09-13 02:43 PM, Vipin Sharma wrote:
> Use MMU shrinker to iterate through all the vCPUs of all the VMs and
> free pages allocated in MMU memory caches. Protect cache allocation in
> page fault and MMU load path from MMU shrinker by using a per vCPU
> mutex. In MMU shrinker, move the iterated VM to the end of the VMs list
> so that the pain of emptying cache spread among other VMs too.
> 
> The specific caches to empty are mmu_shadow_page_cache and
> mmu_shadowed_info_cache as these caches store whole pages. Emptying them
> will give more impact to shrinker compared to other caches like
> mmu_pte_list_desc_cache{} and mmu_page_header_cache{}
> 
> Holding per vCPU mutex lock ensures that a vCPU doesn't get surprised
> by finding its cache emptied after filling them up for page table
> allocations during page fault handling and MMU load operation. Per vCPU
> mutex also makes sure there is only race between MMU shrinker and all
> other vCPUs. This should result in very less contention.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Suggested-by: David Matlack <dmatlack@xxxxxxxxxx>
> Signed-off-by: Vipin Sharma <vipinsh@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  6 +++
>  arch/x86/kvm/mmu/mmu.c          | 69 +++++++++++++++++++++++++++------
>  arch/x86/kvm/mmu/paging_tmpl.h  | 14 ++++---
>  include/linux/kvm_host.h        |  1 +
>  virt/kvm/kvm_main.c             |  8 +++-
>  5 files changed, 81 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index cbfe31bac6cf..63eaf03111eb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -811,6 +811,12 @@ struct kvm_vcpu_arch {
>  	 */
>  	struct kvm_mmu *walk_mmu;
>  
> +	/*
> +	 * Protect cache from getting emptied in MMU shrinker while vCPU might
> +	 * use cache for fault handling or loading MMU.  As this is a per vCPU
> +	 * lock, only contention might happen when MMU shrinker runs.
> +	 */
> +	struct mutex mmu_memory_cache_lock;
>  	struct kvm_mmu_memory_cache mmu_pte_list_desc_cache;
>  	struct kvm_mmu_memory_cache mmu_shadow_page_cache;
>  	struct kvm_mmu_memory_cache mmu_shadowed_info_cache;
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 213e46b55dda..8e2935347615 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4524,29 +4524,33 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  	if (r != RET_PF_INVALID)
>  		return r;
>  
> +	mutex_lock(&vcpu->arch.mmu_memory_cache_lock);
>  	r = mmu_topup_memory_caches(vcpu, false);
>  	if (r)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	r = kvm_faultin_pfn(vcpu, fault, ACC_ALL);
>  	if (r != RET_PF_CONTINUE)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	r = RET_PF_RETRY;
>  	write_lock(&vcpu->kvm->mmu_lock);
>  
>  	if (is_page_fault_stale(vcpu, fault))
> -		goto out_unlock;
> +		goto out_mmu_unlock;
>  
>  	r = make_mmu_pages_available(vcpu);
>  	if (r)
> -		goto out_unlock;
> +		goto out_mmu_unlock;
>  
>  	r = direct_map(vcpu, fault);
>  
> -out_unlock:
> +out_mmu_unlock:
>  	write_unlock(&vcpu->kvm->mmu_lock);
>  	kvm_release_pfn_clean(fault->pfn);
> +out_mmu_memory_cache_unlock:
> +	mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
> +
>  	return r;
>  }
>  
> @@ -4617,25 +4621,28 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,
>  	if (r != RET_PF_INVALID)
>  		return r;
>  
> +	mutex_lock(&vcpu->arch.mmu_memory_cache_lock);
>  	r = mmu_topup_memory_caches(vcpu, false);
>  	if (r)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	r = kvm_faultin_pfn(vcpu, fault, ACC_ALL);
>  	if (r != RET_PF_CONTINUE)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	r = RET_PF_RETRY;
>  	read_lock(&vcpu->kvm->mmu_lock);
>  
>  	if (is_page_fault_stale(vcpu, fault))
> -		goto out_unlock;
> +		goto out_mmu_unlock;
>  
>  	r = kvm_tdp_mmu_map(vcpu, fault);
>  
> -out_unlock:
> +out_mmu_unlock:
>  	read_unlock(&vcpu->kvm->mmu_lock);
>  	kvm_release_pfn_clean(fault->pfn);
> +out_mmu_memory_cache_unlock:
> +	mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
>  	return r;
>  }
>  #endif
> @@ -5691,6 +5698,7 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
>  {
>  	int r;
>  
> +	mutex_lock(&vcpu->arch.mmu_memory_cache_lock);
>  	r = mmu_topup_memory_caches(vcpu, !vcpu->arch.mmu->root_role.direct);
>  	if (r)
>  		goto out;
> @@ -5717,6 +5725,7 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
>  	 */
>  	kvm_x86_call(flush_tlb_current)(vcpu);
>  out:
> +	mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
>  	return r;
>  }
>  
> @@ -6303,6 +6312,7 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
>  	if (!vcpu->arch.mmu_shadow_page_cache.init_value)
>  		vcpu->arch.mmu_shadow_page_cache.gfp_zero = __GFP_ZERO;
>  
> +	mutex_init(&vcpu->arch.mmu_memory_cache_lock);
>  	vcpu->arch.mmu = &vcpu->arch.root_mmu;
>  	vcpu->arch.walk_mmu = &vcpu->arch.root_mmu;
>  
> @@ -6997,13 +7007,50 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm, u64 gen)
>  static unsigned long mmu_shrink_scan(struct shrinker *shrink,
>  				     struct shrink_control *sc)
>  {
> -	return SHRINK_STOP;
> +	struct kvm *kvm, *next_kvm, *first_kvm = NULL;
> +	unsigned long i, freed = 0;
> +	struct kvm_vcpu *vcpu;
> +
> +	mutex_lock(&kvm_lock);
> +	list_for_each_entry_safe(kvm, next_kvm, &vm_list, vm_list) {
> +		if (!first_kvm)
> +			first_kvm = kvm;
> +		else if (first_kvm == kvm)
> +			break;
> +
> +		list_move_tail(&kvm->vm_list, &vm_list);
> +
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			if (!mutex_trylock(&vcpu->arch.mmu_memory_cache_lock))
> +				continue;
> +			freed += kvm_mmu_empty_memory_cache(&vcpu->arch.mmu_shadow_page_cache);
> +			freed += kvm_mmu_empty_memory_cache(&vcpu->arch.mmu_shadowed_info_cache);
> +			mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
> +			if (freed >= sc->nr_to_scan)
> +				goto out;

Looking at the caller in mm/shrinker.c, sc->nr_to_scan will be <= 128
(SHRINK_BATCH), which is only enough for 2 vCPUs. So I think the
shrinker will only ever free 2 vCPU caches of each VM (probably the
first 2 vCPUs) before reordering the list and moving onto the next VM on
the next call.

Does that match the behavior you observe?

> +		}
> +	}
> +out:
> +	mutex_unlock(&kvm_lock);
> +	return freed;
>  }
>  
>  static unsigned long mmu_shrink_count(struct shrinker *shrink,
>  				      struct shrink_control *sc)
>  {
> -	return SHRINK_EMPTY;
> +	unsigned long i, count = 0;
> +	struct kvm_vcpu *vcpu;
> +	struct kvm *kvm;
> +
> +	mutex_lock(&kvm_lock);
> +	list_for_each_entry(kvm, &vm_list, vm_list) {
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			count += READ_ONCE(vcpu->arch.mmu_shadow_page_cache.nobjs);
> +			count += READ_ONCE(vcpu->arch.mmu_shadowed_info_cache.nobjs);
> +		}
> +	}
> +	mutex_unlock(&kvm_lock);
> +	return !count ? SHRINK_EMPTY : count;
>  }
>  
>  static struct shrinker *mmu_shrinker;
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 405bd7ceee2a..084a5c532078 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -809,13 +809,14 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  		return RET_PF_EMULATE;
>  	}
>  
> +	mutex_lock(&vcpu->arch.mmu_memory_cache_lock);
>  	r = mmu_topup_memory_caches(vcpu, true);
>  	if (r)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	r = kvm_faultin_pfn(vcpu, fault, walker.pte_access);
>  	if (r != RET_PF_CONTINUE)
> -		return r;
> +		goto out_mmu_memory_cache_unlock;
>  
>  	/*
>  	 * Do not change pte_access if the pfn is a mmio page, otherwise
> @@ -840,16 +841,19 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>  	write_lock(&vcpu->kvm->mmu_lock);
>  
>  	if (is_page_fault_stale(vcpu, fault))
> -		goto out_unlock;
> +		goto out_mmu_unlock;
>  
>  	r = make_mmu_pages_available(vcpu);
>  	if (r)
> -		goto out_unlock;
> +		goto out_mmu_unlock;
>  	r = FNAME(fetch)(vcpu, fault, &walker);
>  
> -out_unlock:
> +out_mmu_unlock:
>  	write_unlock(&vcpu->kvm->mmu_lock);
>  	kvm_release_pfn_clean(fault->pfn);
> +out_mmu_memory_cache_unlock:
> +	mutex_unlock(&vcpu->arch.mmu_memory_cache_lock);
> +
>  	return r;
>  }
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index b23c6d48392f..288e503f14a0 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1446,6 +1446,7 @@ void kvm_flush_remote_tlbs_memslot(struct kvm *kvm,
>  int kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int min);
>  int __kvm_mmu_topup_memory_cache(struct kvm_mmu_memory_cache *mc, int capacity, int min);
>  int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc);
> +int kvm_mmu_empty_memory_cache(struct kvm_mmu_memory_cache *mc);
>  void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
>  void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
>  #endif
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cb2b78e92910..5d89ca218791 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -451,15 +451,21 @@ int kvm_mmu_memory_cache_nr_free_objects(struct kvm_mmu_memory_cache *mc)
>  	return mc->nobjs;
>  }
>  
> -void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> +int kvm_mmu_empty_memory_cache(struct kvm_mmu_memory_cache *mc)
>  {
> +	int freed = mc->nobjs;
>  	while (mc->nobjs) {
>  		if (mc->kmem_cache)
>  			kmem_cache_free(mc->kmem_cache, mc->objects[--mc->nobjs]);
>  		else
>  			free_page((unsigned long)mc->objects[--mc->nobjs]);
>  	}
> +	return freed;
> +}
>  
> +void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
> +{
> +	kvm_mmu_empty_memory_cache(mc);
>  	kvfree(mc->objects);
>  
>  	mc->objects = NULL;
> -- 
> 2.46.0.662.g92d0881bb0-goog
>