ksm: drain pagevecs to lru

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.

Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: CAI Qian <caiqian@redhat.com>
Cc:Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index 33781de0b6bf..c2b2a94f9d67 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1296,6 +1296,18 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 
         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);