[PATCH] mm: numa: Do not trap faults on shared data section pages.

[Henry Willard <henry.willard@oracle.com>]

Workloads consisting of a large number processes running the same program
with a large shared data section may suffer from excessive numa balancing
page migration of the pages in the shared data section. This shows up as
high I/O wait time and degraded performance on machines with higher socket
or node counts.

This patch skips shared copy-on-write pages in change_pte_range() for the
numa balancing case.

Signed-off-by: Henry Willard <henry.willard@oracle.com>
Reviewed-by: HAJPYkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: Steve Sistare steven.sistare@oracle.com
---
 mm/mprotect.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index ec39f730a0bf..fbbb3ab70818 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -84,6 +84,11 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 				if (!page || PageKsm(page))
 					continue;
 
+				/* Also skip shared copy-on-write pages */
+				if (is_cow_mapping(vma->vm_flags) &&
+				    page_mapcount(page) != 1)
+					continue;
+
 				/* Avoid TLB flush if possible */
 				if (pte_protnone(oldpte))
 					continue;
-- 
1.8.3.1

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