[PATCH 1/3] Properly account for the number of page cache pages zone_reclaim() can reclaim

[Mel Gorman]

On NUMA machines, the administrator can configure zone_reclaim_mode that
is a more targetted form of direct reclaim. On machines with large NUMA
distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
unmapped pages will be reclaimed if the zone watermarks are not being met.

There is a heuristic that determines if the scan is worthwhile but the problem
is that the heuristic is not being properly applied and is basically assuming
zone_reclaim_mode is 1 if it is enabled. The lack of proper detection can
manfiest as high CPU usage as the LRU list is scanned uselessly.

Historically, once enabled it was depending on NR_FILE_PAGES which may
include swapcache pages that the reclaim_mode cannot deal with.  Patch
vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch by
Kosaki Motohiro noted that zone_page_state(zone, NR_FILE_PAGES) included
pages that were not file-backed such as swapcache and made a calculation
based on the inactive, active and mapped files. This is far superior
when zone_reclaim==1 but if RECLAIM_SWAP is set, then NR_FILE_PAGES is a
reasonable starting figure.

This patch alters how zone_reclaim() works out how many pages it might be
able to reclaim given the current reclaim_mode. If RECLAIM_SWAP is set
in the reclaim_mode it will either consider NR_FILE_PAGES as potential
candidates or else use NR_{IN}ACTIVE}_PAGES-NR_FILE_MAPPED to discount
swapcache and other non-file-backed pages.  If RECLAIM_WRITE is not set,
then NR_FILE_DIRTY number of pages are not candidates. If RECLAIM_SWAP is
not set, then NR_FILE_MAPPED are not.

[mmotm note: This patch should be merged with or replace
vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.  Kosaki?]

[kosaki.motohiro@xxxxxxxxxxxxxx: Estimate unmapped pages minus tmpfs pages]
[fengguang.wu@xxxxxxxxx: Fix underflow problem in Kosaki's estimate]
Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
---
 mm/vmscan.c |   55 +++++++++++++++++++++++++++++++++++++++++--------------
 1 files changed, 41 insertions(+), 14 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2ddcfc8..d832ba8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2333,6 +2333,44 @@ int sysctl_min_unmapped_ratio = 1;
  */
 int sysctl_min_slab_ratio = 5;
 
+static inline unsigned long zone_unmapped_file_pages(struct zone *zone)
+{
+	unsigned long file_mapped = zone_page_state(zone, NR_FILE_MAPPED);
+	unsigned long file_lru = zone_page_state(zone, NR_INACTIVE_FILE) +
+		zone_page_state(zone, NR_ACTIVE_FILE);
+
+	/*
+	 * It's possible for there to be more file mapped pages than
+	 * accounted for by the pages on the file LRU lists because
+	 * tmpfs pages accounted for as ANON can also be FILE_MAPPED
+	 */
+	return (file_lru > file_mapped) ? (file_lru - file_mapped) : 0;
+}
+
+/* Work out how many page cache pages we can reclaim in this reclaim_mode */
+static long zone_pagecache_reclaimable(struct zone *zone)
+{
+	long nr_pagecache_reclaimable;
+	long delta = 0;
+
+	/*
+	 * If RECLAIM_SWAP is set, then all file pages are considered
+	 * potentially reclaimable. Otherwise, we have to worry about
+	 * pages like swapcache and zone_unmapped_file_pages() provides
+	 * a better estimate
+	 */
+	if (zone_reclaim_mode & RECLAIM_SWAP)
+		nr_pagecache_reclaimable = zone_page_state(zone, NR_FILE_PAGES);
+	else
+		nr_pagecache_reclaimable = zone_unmapped_file_pages(zone);
+
+	/* If we can't clean pages, remove dirty pages from consideration */
+	if (!(zone_reclaim_mode & RECLAIM_WRITE))
+		delta += zone_page_state(zone, NR_FILE_DIRTY);
+
+	return nr_pagecache_reclaimable;
+}
+
 /*
  * Try to free up some pages from this zone through reclaim.
  */
@@ -2355,7 +2393,6 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 		.isolate_pages = isolate_pages_global,
 	};
 	unsigned long slab_reclaimable;
-	long nr_unmapped_file_pages;
 
 	disable_swap_token();
 	cond_resched();
@@ -2368,11 +2405,7 @@ static int __zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 	reclaim_state.reclaimed_slab = 0;
 	p->reclaim_state = &reclaim_state;
 
-	nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
-				 zone_page_state(zone, NR_ACTIVE_FILE) -
-				 zone_page_state(zone, NR_FILE_MAPPED);
-
-	if (nr_unmapped_file_pages > zone->min_unmapped_pages) {
+	if (zone_pagecache_reclaimable(zone) > zone->min_unmapped_pages) {
 		/*
 		 * Free memory by calling shrink zone with increasing
 		 * priorities until we have enough memory freed.
@@ -2419,8 +2452,6 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 {
 	int node_id;
 	int ret;
-	long nr_unmapped_file_pages;
-	long nr_slab_reclaimable;
 
 	/*
 	 * Zone reclaim reclaims unmapped file backed pages and
@@ -2432,12 +2463,8 @@ int zone_reclaim(struct zone *zone, gfp_t gfp_mask, unsigned int order)
 	 * if less than a specified percentage of the zone is used by
 	 * unmapped file backed pages.
 	 */
-	nr_unmapped_file_pages = zone_page_state(zone, NR_INACTIVE_FILE) +
-				 zone_page_state(zone, NR_ACTIVE_FILE) -
-				 zone_page_state(zone, NR_FILE_MAPPED);
-	nr_slab_reclaimable = zone_page_state(zone, NR_SLAB_RECLAIMABLE);
-	if (nr_unmapped_file_pages <= zone->min_unmapped_pages &&
-	    nr_slab_reclaimable <= zone->min_slab_pages)
+	if (zone_pagecache_reclaimable(zone) <= zone->min_unmapped_pages &&
+	    zone_page_state(zone, NR_SLAB_RECLAIMABLE) <= zone->min_slab_pages)
 		return 0;
 
 	if (zone_is_all_unreclaimable(zone))
-- 
1.5.6.5

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