[PATCH 2/5] mm: vmscan: kick flushers when we encounter dirty pages on the LRU

[Johannes Weiner]

Memory pressure can put dirty pages at the end of the LRU without
anybody running into dirty limits. Don't start writing individual
pages from kswapd while the flushers might be asleep.

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 include/linux/writeback.h        |  2 +-
 include/trace/events/writeback.h |  2 +-
 mm/vmscan.c                      | 18 +++++++++++++-----
 3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 5527d910ba3d..a3c0cbd7c888 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -46,7 +46,7 @@ enum writeback_sync_modes {
  */
 enum wb_reason {
 	WB_REASON_BACKGROUND,
-	WB_REASON_TRY_TO_FREE_PAGES,
+	WB_REASON_VMSCAN,
 	WB_REASON_SYNC,
 	WB_REASON_PERIODIC,
 	WB_REASON_LAPTOP_TIMER,
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 2ccd9ccbf9ef..7bd8783a590f 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -31,7 +31,7 @@
 
 #define WB_WORK_REASON							\
 	EM( WB_REASON_BACKGROUND,		"background")		\
-	EM( WB_REASON_TRY_TO_FREE_PAGES,	"try_to_free_pages")	\
+	EM( WB_REASON_VMSCAN,			"vmscan")		\
 	EM( WB_REASON_SYNC,			"sync")			\
 	EM( WB_REASON_PERIODIC,			"periodic")		\
 	EM( WB_REASON_LAPTOP_TIMER,		"laptop_timer")		\
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0d05f7f3b532..56ea8d24041f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1798,12 +1798,20 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 
 		/*
 		 * If dirty pages are scanned that are not queued for IO, it
-		 * implies that flushers are not keeping up. In this case, flag
-		 * the pgdat PGDAT_DIRTY and kswapd will start writing pages from
-		 * reclaim context.
+		 * implies that flushers are not doing their job. This can
+		 * happen when memory pressure pushes dirty pages to the end
+		 * of the LRU without the dirty limits being breached. It can
+		 * also happen when the proportion of dirty pages grows not
+		 * through writes but through memory pressure reclaiming all
+		 * the clean cache. And in some cases, the flushers simply
+		 * cannot keep up with the allocation rate. Nudge the flusher
+		 * threads in case they are asleep, but also allow kswapd to
+		 * start writing pages during reclaim.
 		 */
-		if (stat.nr_unqueued_dirty == nr_taken)
+		if (stat.nr_unqueued_dirty == nr_taken) {
+			wakeup_flusher_threads(0, WB_REASON_VMSCAN);
 			set_bit(PGDAT_DIRTY, &pgdat->flags);
+		}
 
 		/*
 		 * If kswapd scans pages marked marked for immediate
@@ -2787,7 +2795,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
 		writeback_threshold = sc->nr_to_reclaim + sc->nr_to_reclaim / 2;
 		if (total_scanned > writeback_threshold) {
 			wakeup_flusher_threads(laptop_mode ? 0 : total_scanned,
-						WB_REASON_TRY_TO_FREE_PAGES);
+						WB_REASON_VMSCAN);
 			sc->may_writepage = 1;
 		}
 	} while (--sc->priority >= 0);
-- 
2.11.0