mm: more likely reclaim MADV_SEQUENTIAL mappings

File pages mapped only in sequentially read mappings are perfect reclaim canditates.

This patch makes these mappings behave like weak references, their pages will be reclaimed unless they have a strong reference from a normal mapping as well.

It changes the reclaim and the unmap path where they check if the page has been referenced. In both cases, accesses through sequentially read mappings will be ignored.

Signed-off-by Johannes Weiner <[email protected]>

Signed-off-by Rik van Riel <[email protected]>

Acked-by KOSAKI Motohiro <[email protected]>

CC Nick Piggin <[email protected]>

---

II: add likely()s to mitigate the extra branches a bit as to Nick's
suggestion

Benchmark results from KOSAKI Motohiro:

    http://marc.info/?l=linux-mm&m=122485301925098&w=2

 mm/memory.c |    3 ++-
 mm/rmap.c   |   13 +++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

mm/rmap.c +11 -2

@@ -337,8 +337,17 @@ static int page_referenced_one(struct pa
 		goto out_unmap;
 	}
 
-	if (ptep_clear_flush_young_notify(vma, address, pte))
-		referenced++;
+	if (ptep_clear_flush_young_notify(vma, address, pte)) {
+		/*
+		 * Don't treat a reference through a sequentially read
+		 * mapping as such.  If the page has been used in
+		 * another mapping, we will catch it; if this other
+		 * mapping is already gone, the unmap path will have
+		 * set PG_referenced or activated the page.
+		 */
+		if (likely(!VM_SequentialReadHint(vma)))
+			referenced++;
+	}
 
 	/* Pretend the page is referenced if the task has the
 	   swap token and is in the middle of a page fault. */