[RFC PATCH 14/17] KVM: arm64: Punt last page reference to rcu callback for parallel walk

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It is possible that a table page remains visible to another thread until
the next rcu synchronization event. To that end, we cannot drop the last
page reference synchronous with post-order traversal for a parallel
table walk.

Schedule an rcu callback to clean up the child table page for parallel
walks.

Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
---
 arch/arm64/include/asm/kvm_pgtable.h |  3 ++
 arch/arm64/kvm/hyp/pgtable.c         | 24 +++++++++++++--
 arch/arm64/kvm/mmu.c                 | 44 +++++++++++++++++++++++++++-
 3 files changed, 67 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 74955aba5918..52e55e00f0ca 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -81,6 +81,8 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
  * @put_page:			Decrement the refcount on a page. When the
  *				refcount reaches 0 the page is automatically
  *				freed.
+ * @free_table:			Drop the last page reference, possibly in the
+ *				next RCU sync if doing a shared walk.
  * @page_count:			Return the refcount of a page.
  * @phys_to_virt:		Convert a physical address into a virtual
  *				address	mapped in the current context.
@@ -98,6 +100,7 @@ struct kvm_pgtable_mm_ops {
 	void		(*get_page)(void *addr);
 	void		(*put_page)(void *addr);
 	int		(*page_count)(void *addr);
+	void		(*free_table)(void *addr, bool shared);
 	void*		(*phys_to_virt)(phys_addr_t phys);
 	phys_addr_t	(*virt_to_phys)(void *addr);
 	void		(*dcache_clean_inval_poc)(void *addr, size_t size);
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 121818d4c33e..a9a48edba63b 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -147,12 +147,19 @@ static inline void kvm_pgtable_walk_end(void)
 {}
 
 #define kvm_dereference_ptep	rcu_dereference_raw
+
+static inline void kvm_pgtable_destroy_barrier(void)
+{}
+
 #else
 #define kvm_pgtable_walk_begin	rcu_read_lock
 
 #define kvm_pgtable_walk_end	rcu_read_unlock
 
 #define kvm_dereference_ptep	rcu_dereference
+
+#define kvm_pgtable_destroy_barrier	rcu_barrier
+
 #endif
 
 static kvm_pte_t *kvm_pte_follow(kvm_pte_t pte, struct kvm_pgtable_mm_ops *mm_ops)
@@ -1063,7 +1070,12 @@ static int stage2_map_walk_table_post(u64 addr, u64 end, u32 level,
 		childp = kvm_pte_follow(*old, mm_ops);
 	}
 
-	mm_ops->put_page(childp);
+	/*
+	 * If we do not have exclusive access to the page tables it is possible
+	 * the unlinked table remains visible to another thread until the next
+	 * rcu synchronization.
+	 */
+	mm_ops->free_table(childp, shared);
 	mm_ops->put_page(ptep);
 
 	return ret;
@@ -1203,7 +1215,7 @@ static int stage2_unmap_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
 					       kvm_granule_size(level));
 
 	if (childp)
-		mm_ops->put_page(childp);
+		mm_ops->free_table(childp, shared);
 
 	return 0;
 }
@@ -1433,7 +1445,7 @@ static int stage2_free_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
 	mm_ops->put_page(ptep);
 
 	if (kvm_pte_table(*old, level))
-		mm_ops->put_page(kvm_pte_follow(*old, mm_ops));
+		mm_ops->free_table(kvm_pte_follow(*old, mm_ops), shared);
 
 	return 0;
 }
@@ -1452,4 +1464,10 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt)
 	pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level) * PAGE_SIZE;
 	pgt->mm_ops->free_pages_exact(pgt->pgd, pgd_sz);
 	pgt->pgd = NULL;
+
+	/*
+	 * Guarantee that all unlinked subtrees associated with the stage2 page
+	 * table have also been freed before returning.
+	 */
+	kvm_pgtable_destroy_barrier();
 }
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index cc6ed6b06ec2..6ecf37009c21 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -98,9 +98,50 @@ static bool kvm_is_device_pfn(unsigned long pfn)
 static void *stage2_memcache_zalloc_page(void *arg)
 {
 	struct kvm_mmu_caches *mmu_caches = arg;
+	struct stage2_page_header *hdr;
+	void *addr;
 
 	/* Allocated with __GFP_ZERO, so no need to zero */
-	return kvm_mmu_memory_cache_alloc(&mmu_caches->page_cache);
+	addr = kvm_mmu_memory_cache_alloc(&mmu_caches->page_cache);
+	if (!addr)
+		return NULL;
+
+	hdr = kvm_mmu_memory_cache_alloc(&mmu_caches->header_cache);
+	if (!hdr) {
+		free_page((unsigned long)addr);
+		return NULL;
+	}
+
+	hdr->page = virt_to_page(addr);
+	set_page_private(hdr->page, (unsigned long)hdr);
+	return addr;
+}
+
+static void stage2_free_page_now(struct stage2_page_header *hdr)
+{
+	WARN_ON(page_ref_count(hdr->page) != 1);
+
+	__free_page(hdr->page);
+	kmem_cache_free(stage2_page_header_cache, hdr);
+}
+
+static void stage2_free_page_rcu_cb(struct rcu_head *head)
+{
+	struct stage2_page_header *hdr = container_of(head, struct stage2_page_header,
+						      rcu_head);
+
+	stage2_free_page_now(hdr);
+}
+
+static void stage2_free_table(void *addr, bool shared)
+{
+	struct page *page = virt_to_page(addr);
+	struct stage2_page_header *hdr = (struct stage2_page_header *)page_private(page);
+
+	if (shared)
+		call_rcu(&hdr->rcu_head, stage2_free_page_rcu_cb);
+	else
+		stage2_free_page_now(hdr);
 }
 
 static void *kvm_host_zalloc_pages_exact(size_t size)
@@ -613,6 +654,7 @@ static struct kvm_pgtable_mm_ops kvm_s2_mm_ops = {
 	.free_pages_exact	= free_pages_exact,
 	.get_page		= kvm_host_get_page,
 	.put_page		= kvm_host_put_page,
+	.free_table		= stage2_free_table,
 	.page_count		= kvm_host_page_count,
 	.phys_to_virt		= kvm_host_va,
 	.virt_to_phys		= kvm_host_pa,
-- 
2.36.0.rc0.470.gd361397f0d-goog




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