Re: [PATCH 09/14] KVM: arm64: Free removed stage-2 tables in RCU callback

[David Matlack]

On Tue, Aug 30, 2022 at 07:41:27PM +0000, Oliver Upton wrote:
> There is no real urgency to free a stage-2 subtree that was pruned.
> Nonetheless, KVM does the tear down in the stage-2 fault path while
> holding the MMU lock.
> 
> Free removed stage-2 subtrees after an RCU grace period. To guarantee
> all stage-2 table pages are freed before killing a VM, add an
> rcu_barrier() to the flush path.
> 
> Signed-off-by: Oliver Upton <oliver.upton@xxxxxxxxx>
> ---
>  arch/arm64/kvm/mmu.c | 35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 91521f4aab97..265951c05879 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -97,6 +97,38 @@ static void *stage2_memcache_zalloc_page(void *arg)
>  	return kvm_mmu_memory_cache_alloc(mc);
>  }
>  
> +#define STAGE2_PAGE_PRIVATE_LEVEL_MASK	GENMASK_ULL(2, 0)
> +
> +static inline unsigned long stage2_page_private(u32 level, void *arg)
> +{
> +	unsigned long pvt = (unsigned long)arg;
> +
> +	BUILD_BUG_ON(KVM_PGTABLE_MAX_LEVELS > STAGE2_PAGE_PRIVATE_LEVEL_MASK);
> +	WARN_ON_ONCE(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK);
> +
> +	return pvt | level;
> +}
> +
> +static void stage2_free_removed_table_rcu_cb(struct rcu_head *head)
> +{
> +	struct page *page = container_of(head, struct page, rcu_head);
> +	unsigned long pvt = page_private(page);
> +	void *arg = (void *)(pvt & ~STAGE2_PAGE_PRIVATE_LEVEL_MASK);
> +	u32 level = (u32)(pvt & STAGE2_PAGE_PRIVATE_LEVEL_MASK);
> +	void *pgtable = page_to_virt(page);
> +
> +	kvm_pgtable_stage2_free_removed(pgtable, level, arg);
> +}
> +
> +static void stage2_free_removed_table(void *pgtable, u32 level, void *arg)
> +{
> +	unsigned long pvt = stage2_page_private(level, arg);
> +	struct page *page = virt_to_page(pgtable);
> +
> +	set_page_private(page, (unsigned long)pvt);
> +	call_rcu(&page->rcu_head, stage2_free_removed_table_rcu_cb);
> +}
> +
>  static void *kvm_host_zalloc_pages_exact(size_t size)
>  {
>  	return alloc_pages_exact(size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> @@ -627,7 +659,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
>  	.zalloc_page		= stage2_memcache_zalloc_page,
>  	.zalloc_pages_exact	= kvm_host_zalloc_pages_exact,
>  	.free_pages_exact	= free_pages_exact,
> -	.free_removed_table	= kvm_pgtable_stage2_free_removed,
> +	.free_removed_table	= stage2_free_removed_table,
>  	.get_page		= kvm_host_get_page,
>  	.put_page		= kvm_host_put_page,
>  	.page_count		= kvm_host_page_count,
> @@ -770,6 +802,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
>  	if (pgt) {
>  		kvm_pgtable_stage2_destroy(pgt);
>  		kfree(pgt);
> +		rcu_barrier();

A comment here would be useful to document the behavior. e.g.

        /*
         * Wait for all stage-2 page tables that are being freed
         * asynchronously via RCU callback because ...
         */

Speaking of, what's the reason for this rcu_barrier()? Is there any
reason why KVM can't let in-flight stage-2 freeing RCU callbacks run at
the end of the next grace period?

>  	}
>  }
>  
> -- 
> 2.37.2.672.g94769d06f0-goog
>