Linux-mm Archive on
 help / color / Atom feed
* [patch 035/158] mm/rmap.c: reuse mergeable anon_vma as parent when fork
@ 2019-12-01  1:50 akpm
  0 siblings, 0 replies; only message in thread
From: akpm @ 2019-12-01  1:50 UTC (permalink / raw)
  To: akpm, cai, jglisse, khlebnikov, kirill.shutemov, linux-mm,
	mike.kravetz, mm-commits, richardw.yang, riel, shakeelb,

From: Wei Yang <>
Subject: mm/rmap.c: reuse mergeable anon_vma as parent when fork

In __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 <>
Acked-by: Konstantin Khlebnikov <>
Cc: Kirill A. Shutemov <>
Cc: "Jérôme Glisse" <>
Cc: Mike Kravetz <>
Cc: Rik van Riel <>
Cc: Qian Cai <>
Cc: Shakeel Butt <>
Signed-off-by: Andrew Morton <>

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

--- a/mm/rmap.c~mm-rmapc-reuse-mergeable-anon_vma-as-parent-when-fork
+++ a/mm/rmap.c
@@ -269,6 +269,19 @@ int anon_vma_clone(struct vm_area_struct
 	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;
 	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma;

^ permalink raw reply	[flat|nested] only message in thread

only message in thread, back to index

Thread overview: (only message) (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-12-01  1:50 [patch 035/158] mm/rmap.c: reuse mergeable anon_vma as parent when fork akpm

Linux-mm Archive on

Archives are clonable:
	git clone --mirror linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ \
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone