[patch 3/3] vmscan: detect mapped file pages used only once

[Johannes Weiner <hannes@cmpxchg.org>]

The VM currently assumes that an inactive, mapped and referenced file
page is in use and promotes it to the active list.

However, every mapped file page starts out like this and thus a problem
arises when workloads create a stream of such pages that are used only
for a short time.  By flooding the active list with those pages, the VM
quickly gets into trouble finding eligible reclaim canditates.  The
result is long allocation latencies and eviction of the wrong pages.

This patch reuses the PG_referenced page flag (used for unmapped file
pages) to implement a usage detection that scales with the speed of
LRU list cycling (i.e. memory pressure).

If the scanner encounters those pages, the flag is set and the page
cycled again on the inactive list.  Only if it returns with another
page table reference it is activated.  Otherwise it is reclaimed as
'not recently used cache'.

This effectively changes the minimum lifetime of a used-once mapped
file page from a full memory cycle to an inactive list cycle, which
allows it to occur in linear streams without affecting the stable
working set of the system.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/rmap.h |    2 +-
 mm/rmap.c            |    3 ---
 mm/vmscan.c          |   45 +++++++++++++++++++++++++++++++++++----------
 3 files changed, 36 insertions(+), 14 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index b019ae6..f4accb5 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -181,7 +181,7 @@ static inline int page_referenced(struct page *page, int is_locked,
 				  unsigned long *vm_flags)
 {
 	*vm_flags = 0;
-	return TestClearPageReferenced(page);
+	return 0;
 }
 
 #define try_to_unmap(page, refs) SWAP_FAIL
diff --git a/mm/rmap.c b/mm/rmap.c
index 278cd27..5a48bda 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -511,9 +511,6 @@ int page_referenced(struct page *page,
 	int referenced = 0;
 	int we_locked = 0;
 
-	if (TestClearPageReferenced(page))
-		referenced++;
-
 	*vm_flags = 0;
 	if (page_mapped(page) && page_rmapping(page)) {
 		if (!is_locked && (!PageAnon(page) || PageKsm(page))) {
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a8e4cbe..674a78b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -561,18 +561,18 @@ redo:
 enum page_references {
 	PAGEREF_RECLAIM,
 	PAGEREF_RECLAIM_CLEAN,
+	PAGEREF_KEEP,
 	PAGEREF_ACTIVATE,
 };
 
 static enum page_references page_check_references(struct page *page,
 						  struct scan_control *sc)
 {
+	int referenced_ptes, referenced_page;
 	unsigned long vm_flags;
-	int referenced;
 
-	referenced = page_referenced(page, 1, sc->mem_cgroup, &vm_flags);
-	if (!referenced)
-		return PAGEREF_RECLAIM;
+	referenced_ptes = page_referenced(page, 1, sc->mem_cgroup, &vm_flags);
+	referenced_page = TestClearPageReferenced(page);
 
 	/* Lumpy reclaim - ignore references */
 	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
@@ -582,11 +582,36 @@ static enum page_references page_check_references(struct page *page,
 	if (vm_flags & VM_LOCKED)
 		return PAGEREF_RECLAIM;
 
-	if (page_mapped(page))
-		return PAGEREF_ACTIVATE;
+	if (referenced_ptes) {
+		if (PageAnon(page))
+			return PAGEREF_ACTIVATE;
+		/*
+		 * All mapped pages start out with page table
+		 * references from the instantiating fault, so we need
+		 * to look twice if a mapped file page is used more
+		 * than once.
+		 *
+		 * Mark it and spare it for another trip around the
+		 * inactive list.  Another page table reference will
+		 * lead to its activation.
+		 *
+		 * Note: the mark is set for activated pages as well
+		 * so that recently deactivated but used pages are
+		 * quickly recovered.
+		 */
+		SetPageReferenced(page);
+
+		if (referenced_page)
+			return PAGEREF_ACTIVATE;
+
+		return PAGEREF_KEEP;
+	}
 
 	/* Reclaim if clean, defer dirty pages to writeback */
-	return PAGEREF_RECLAIM_CLEAN;
+	if (referenced_page)
+		return PAGEREF_RECLAIM_CLEAN;
+
+	return PAGEREF_RECLAIM;
 }
 
 /*
@@ -654,6 +679,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		switch (references) {
 		case PAGEREF_ACTIVATE:
 			goto activate_locked;
+		case PAGEREF_KEEP:
+			goto keep_locked;
 		case PAGEREF_RECLAIM:
 		case PAGEREF_RECLAIM_CLEAN:
 			; /* try to reclaim the page below */
@@ -1356,9 +1383,7 @@ static void shrink_active_list(unsigned long nr_pages, struct zone *zone,
 			continue;
 		}
 
-		/* page_referenced clears PageReferenced */
-		if (page_mapped(page) &&
-		    page_referenced(page, 0, sc->mem_cgroup, &vm_flags)) {
+		if (page_referenced(page, 0, sc->mem_cgroup, &vm_flags)) {
 			nr_rotated++;
 			/*
 			 * Identify referenced, file-backed active pages and
-- 
1.6.6.1

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