mm: reuse old anon_vma if it's lost all vmas

From: Konstantin Khlebnikov <koct9i@gmail.com>

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
---
 include/linux/rmap.h |    2 ++
 mm/rmap.c            |   14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index c0c2bce..d40ca08 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -36,6 +36,8 @@ struct anon_vma {
 	 */
 	atomic_t refcount;
 
+	int nr_vmas;	/* Number of direct references from vmas */
+
 	/*
 	 * NOTE: the LSB of the rb_root.rb_node is set by
 	 * mm_take_all_locks() _after_ taking the above lock. So the
diff --git a/mm/rmap.c b/mm/rmap.c
index 19886fb..ced4754 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -72,6 +72,7 @@ static inline struct anon_vma *anon_vma_alloc(void)
 	anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
 	if (anon_vma) {
 		atomic_set(&anon_vma->refcount, 1);
+		anon_vma->nr_vmas = 1;
 		/*
 		 * Initialise the anon_vma root to point to itself. If called
 		 * from fork, the root will be reset to the parents anon_vma.
@@ -256,7 +257,11 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 		anon_vma = pavc->anon_vma;
 		root = lock_anon_vma_root(root, anon_vma);
 		anon_vma_chain_link(dst, avc, anon_vma);
+		if (!dst->anon_vma && !anon_vma->nr_vmas)
+			dst->anon_vma = anon_vma;
 	}
+	if (dst->anon_vma)
+		dst->anon_vma->nr_vmas++;
 	unlock_anon_vma_root(root);
 	return 0;
 
@@ -279,6 +284,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	if (!pvma->anon_vma)
 		return 0;
 
+	/* Drop parent anon_vma, we want find or allocate our own. */
+	vma->anon_vma = NULL;
+
 	/*
 	 * First, attach the new VMA to the parent VMA's anon_vmas,
 	 * so rmap can find non-COWed pages in child processes.
@@ -286,6 +294,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
 	if (anon_vma_clone(vma, pvma))
 		return -ENOMEM;
 
+	/* Old anon_vma has been reused. */
+	if (vma->anon_vma)
+		return 0;
+
 	/* Then add our own anon_vma. */
 	anon_vma = anon_vma_alloc();
 	if (!anon_vma)
@@ -345,6 +357,8 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
+	if (vma->anon_vma)
+		vma->anon_vma->nr_vmas--;
 	unlock_anon_vma_root(root);
 
 	/*