Re: [Patch v4 2/2] mm/rmap.c: reuse mergeable anon_vma as parent when fork

From: Konstantin Khlebnikov
Date: Fri Oct 11 2019 - 04:10:42 EST

On 11/10/2019 10.22, Wei Yang wrote:
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 11.6%.


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


real 2m39.933s
user 17m1.835s
sys 1m38.802s

real 2m39.321s
user 17m1.634s
sys 1m39.206s

real 2m39.575s
user 17m1.420s
sys 1m38.845s

Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>

Acked-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

mm/rmap.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index c34414567474..2c13e2bfd393 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -268,6 +268,19 @@ 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 (!dst->anon_vma && src->anon_vma &&
+ pprev && pprev->anon_vma == src->anon_vma)
+ dst->anon_vma = prev->anon_vma;

I believe that in present code "prev" cannot be NULL if !dst->anon_vma && src->anon_vma is true.
It would be safer to check this explicitly.

list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
struct anon_vma *anon_vma;