From 7a3ef208e662f4b63d43a23f61a64a129c525bbc Mon Sep 17 00:00:00 2001 From: Konstantin Khlebnikov Date: Thu, 8 Jan 2015 14:32:15 -0800 Subject: mm: prevent endless growth of anon_vma hierarchy Constantly forking task causes unlimited grow of anon_vma chain. Each next child allocates new level of anon_vmas and links vma to all previous levels because pages might be inherited from any level. This patch adds heuristic which decides to reuse existing anon_vma instead of forking new one. It adds counter anon_vma->degree which counts linked vmas and directly descending anon_vmas and reuses anon_vma if counter is lower than two. As a result each anon_vma has either vma or at least two descending anon_vmas. In such trees half of nodes are leafs with alive vmas, thus count of anon_vmas is no more than two times bigger than count of vmas. This heuristic reuses anon_vmas as few as possible because each reuse adds false aliasing among vmas and rmap walker ought to scan more ptes when it searches where page is might be mapped. Link: http://lkml.kernel.org/r/20120816024610.GA5350@evergreen.ssec.wisc.edu Fixes: 5beb49305251 ("mm: change anon_vma linking to fix multi-process server scalability issue") [akpm@linux-foundation.org: fix typo, per Rik] Signed-off-by: Konstantin Khlebnikov Reported-by: Daniel Forrest Tested-by: Michal Hocko Tested-by: Jerome Marchand Reviewed-by: Michal Hocko Reviewed-by: Rik van Riel Cc: [2.6.34+] Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/rmap.h | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'include/linux') diff --git a/include/linux/rmap.h b/include/linux/rmap.h index c0c2bce6b0b7..d9d7e7e56352 100644 --- a/include/linux/rmap.h +++ b/include/linux/rmap.h @@ -36,6 +36,16 @@ struct anon_vma { */ atomic_t refcount; + /* + * Count of child anon_vmas and VMAs which points to this anon_vma. + * + * This counter is used for making decision about reusing anon_vma + * instead of forking new one. See comments in function anon_vma_clone. + */ + unsigned degree; + + struct anon_vma *parent; /* Parent of this anon_vma */ + /* * NOTE: the LSB of the rb_root.rb_node is set by * mm_take_all_locks() _after_ taking the above lock. So the -- cgit v1.2.3