[PATCH 5/8] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority

[Mel Gorman]

It is preferable that no dirty pages are dispatched for cleaning from
the page reclaim path. At normal priorities, this patch prevents kswapd
writing pages.

However, page reclaim does have a requirement that pages be freed
in a particular zone. If it is failing to make sufficient progress
(reclaiming < SWAP_CLUSTER_MAX at any priority priority), the priority
is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
considered to tbe the point where kswapd is getting into trouble
reclaiming pages. If this priority is reached, kswapd will dispatch
pages for writing.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/vmscan.c |   13 ++++++++-----
 1 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index ee00c94..cf7b501 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -719,7 +719,8 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
  */
 static unsigned long shrink_page_list(struct list_head *page_list,
 				      struct zone *zone,
-				      struct scan_control *sc)
+				      struct scan_control *sc,
+				      int priority)
 {
 	LIST_HEAD(ret_pages);
 	LIST_HEAD(free_pages);
@@ -827,9 +828,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 
 			/*
 			 * Only kswapd can writeback filesystem pages to
-			 * avoid risk of stack overflow
+			 * avoid risk of stack overflow but do not writeback
+			 * unless under significant pressure.
 			 */
-			if (page_is_file_cache(page) && !current_is_kswapd()) {
+			if (page_is_file_cache(page) &&
+					(!current_is_kswapd() || priority >= DEF_PRIORITY - 2)) {
 				inc_zone_page_state(page, NR_VMSCAN_WRITE_SKIP);
 				goto keep_locked;
 			}
@@ -1465,12 +1468,12 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
 
 	spin_unlock_irq(&zone->lru_lock);
 
-	nr_reclaimed = shrink_page_list(&page_list, zone, sc);
+	nr_reclaimed = shrink_page_list(&page_list, zone, sc, priority);
 
 	/* Check if we should syncronously wait for writeback */
 	if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {
 		set_reclaim_mode(priority, sc, true);
-		nr_reclaimed += shrink_page_list(&page_list, zone, sc);
+		nr_reclaimed += shrink_page_list(&page_list, zone, sc, priority);
 	}
 
 	local_irq_disable();
-- 
1.7.3.4

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