[PATCH 2/5] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority

[Mel Gorman <mgorman@xxxxxxx>]

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 2d3e5b6..e272951 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