[Patch v3 2/2] mm/rmap.c: reuse mergeable anon_vma as parent when fork

[Wei Yang <richardw.yang@linux.intel.com>]

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.

Do the same kernel build test, it shows run time in sys reduced 11.5%.

Origin:

real    2m50.467s
user    17m52.002s
sys     1m51.953s

real    2m48.662s
user    17m55.464s
sys     1m50.553s

real    2m51.143s
user    17m59.687s
sys     1m53.600s

Patched:

real	2m40.080s
user	17m4.644s
sys	1m39.321s

real	2m39.967s
user	17m2.445s
sys	1m38.850s

real	2m40.581s
user	17m1.975s
sys	1m39.065s

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 mm/rmap.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index fc0aba7fb9b9..0dd5f8b04a48 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -269,6 +269,18 @@ 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;
 	bool reuse = !dst->anon_vma && src->anon_vma;
+	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 (reuse && 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