[PATCH] ksm: drain pagevecs to lru

[Hugh Dickins <hughd@google.com>]

It was hard to explain the page counts which were causing new LTP tests
of KSM to fail: we need to drain the per-cpu pagevecs to LRU occasionally.

Reported-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/ksm.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- 2.6.37/mm/ksm.c	2010-12-24 19:31:45.000000000 -0800
+++ linux/mm/ksm.c	2011-01-02 15:06:52.000000000 -0800
@@ -1247,6 +1247,18 @@ static struct rmap_item *scan_get_next_r
 
 	slot = ksm_scan.mm_slot;
 	if (slot == &ksm_mm_head) {
+		/*
+		 * A number of pages can hang around indefinitely on per-cpu
+		 * pagevecs, raised page count preventing write_protect_page
+		 * from merging them.  Though it doesn't really matter much,
+		 * it is puzzling to see some stuck in pages_volatile until
+		 * other activity jostles them out, and they also prevented
+		 * LTP's KSM test from succeeding deterministically; so drain
+		 * them here (here rather than on entry to ksm_do_scan(),
+		 * so we don't IPI too often when pages_to_scan is set low).
+		 */
+		lru_add_drain_all();
+
 		root_unstable_tree = RB_ROOT;
 
 		spin_lock(&ksm_mmlist_lock);

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