vmscan: cleanup the scan batching code

The vmscan batching logic is twisting.  Move it into a standalone function
nr_scan_try_batch() and document it.  No behavior change.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 33 insertions, 16 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 131655cdb6b2..e5b8f628d166 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3657,7 +3657,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat,
                 zone_pcp_init(zone);
                 for_each_lru(l) {
                         INIT_LIST_HEAD(&zone->lru[l].list);
-                        zone->lru[l].nr_scan = 0;
+                        zone->lru[l].nr_saved_scan = 0;
                 }
                 zone->reclaim_stat.recent_rotated[0] = 0;
                 zone->reclaim_stat.recent_rotated[1] = 0;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9673437a5457..d4da097533ce 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1492,6 +1492,26 @@ static void get_scan_ratio(struct zone *zone, struct scan_control *sc,
         percent[1] = 100 - percent[0];
 }
 
+/*
+ * Smallish @nr_to_scan's are deposited in @nr_saved_scan,
+ * until we collected @swap_cluster_max pages to scan.
+ */
+static unsigned long nr_scan_try_batch(unsigned long nr_to_scan,
+                                       unsigned long *nr_saved_scan,
+                                       unsigned long swap_cluster_max)
+{
+        unsigned long nr;
+
+        *nr_saved_scan += nr_to_scan;
+        nr = *nr_saved_scan;
+
+        if (nr >= swap_cluster_max)
+                *nr_saved_scan = 0;
+        else
+                nr = 0;
+
+        return nr;
+}
 
 /*
  * This is a basic per-zone page freer.  Used by both kswapd and direct reclaim.
@@ -1517,14 +1537,11 @@ static void shrink_zone(int priority, struct zone *zone,
                         scan >>= priority;
                         scan = (scan * percent[file]) / 100;
                 }
-                if (scanning_global_lru(sc)) {
-                        zone->lru[l].nr_scan += scan;
-                        nr[l] = zone->lru[l].nr_scan;
-                        if (nr[l] >= swap_cluster_max)
-                                zone->lru[l].nr_scan = 0;
-                        else
-                                nr[l] = 0;
-                } else
+                if (scanning_global_lru(sc))
+                        nr[l] = nr_scan_try_batch(scan,
+                                                  &zone->lru[l].nr_saved_scan,
+                                                  swap_cluster_max);
+                else
                         nr[l] = scan;
         }
 
@@ -2124,11 +2141,11 @@ static void shrink_all_zones(unsigned long nr_pages, int prio,
                                                 l == LRU_ACTIVE_FILE))
                                 continue;
 
-                        zone->lru[l].nr_scan += (lru_pages >> prio) + 1;
-                        if (zone->lru[l].nr_scan >= nr_pages || pass > 3) {
+                        zone->lru[l].nr_saved_scan += (lru_pages >> prio) + 1;
+                        if (zone->lru[l].nr_saved_scan >= nr_pages || pass > 3) {
                                 unsigned long nr_to_scan;
 
-                                zone->lru[l].nr_scan = 0;
+                                zone->lru[l].nr_saved_scan = 0;
                                 nr_to_scan = min(nr_pages, lru_pages);
                                 nr_reclaimed += shrink_list(l, nr_to_scan, zone,
                                                                 sc, prio);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 415110772c73..84c055556911 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -718,10 +718,10 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
                    low_wmark_pages(zone),
                    high_wmark_pages(zone),
                    zone->pages_scanned,
-                   zone->lru[LRU_ACTIVE_ANON].nr_scan,
-                   zone->lru[LRU_INACTIVE_ANON].nr_scan,
-                   zone->lru[LRU_ACTIVE_FILE].nr_scan,
-                   zone->lru[LRU_INACTIVE_FILE].nr_scan,
+                   zone->lru[LRU_ACTIVE_ANON].nr_saved_scan,
+                   zone->lru[LRU_INACTIVE_ANON].nr_saved_scan,
+                   zone->lru[LRU_ACTIVE_FILE].nr_saved_scan,
+                   zone->lru[LRU_INACTIVE_FILE].nr_saved_scan,
                    zone->spanned_pages,
                    zone->present_pages);