[PATCH] mm/rmap.c: reuse mergeable anon_vma as parent when fork

[Wei Yang]

In function __anon_vma_prepare(), we will try to find anon_vma if it is
possible to reuse it. While on fork, the logic is different.

Since commit 5beb49305251 ("mm: change anon_vma linking to fix
multi-process server scalability issue"), function anon_vma_clone()
tries to allocate new anon_vma for child process. But the logic here
will allocate a new anon_vma for each vma, even in parent this vma
is mergeable and share the same anon_vma with its sibling. This may do
better for scalability issue, while it is not necessary to do so
especially after interval tree is used.

Commit 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
tries to reuse some anon_vma by counting child anon_vma and attached
vmas. While for those mergeable anon_vmas, we can just reuse it and not
necessary to go through the logic.

After this change, kernel build test reduces 20% anon_vma allocation.

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
---
 mm/rmap.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index d9a23bb773bf..12f6c3d7fd9d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -262,6 +262,17 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 {
 	struct anon_vma_chain *avc, *pavc;
 	struct anon_vma *root = NULL;
+	struct vm_area_struct *prev = dst->vm_prev, *pprev = src->vm_prev;
+
+	/*
+	 * If parent share anon_vma with its vm_prev, keep this sharing in in
+	 * child.
+	 *
+	 * 1. Parent has vm_prev, which implies we have vm_prev.
+	 * 2. Parent and its vm_prev have the same anon_vma.
+	 */
+	if (pprev && pprev->anon_vma == src->anon_vma)
+		dst->anon_vma = prev->anon_vma;
 
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;
-- 
2.17.1