mm: vmscan: clean up get_scan_count()

Reclaim pressure balance between anon and file pages is calculated
through a tuple of numerators and a shared denominator.

Exceptional cases that want to force-scan anon or file pages configure
the numerators and denominator such that one list is preferred, which is
not necessarily the most obvious way:

    fraction[0] = 1;
    fraction[1] = 0;
    denominator = 1;
    goto out;

Make this easier by making the force-scan cases explicit and use the
fractionals only in case they are calculated from reclaim history.

[akpm@linux-foundation.org: avoid using unintialized_var()]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Satoru Moriya <satoru.moriya@hds.com>
Cc: Simon Jeons <simon.jeons@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 44 insertions, 21 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 259f8208a388..d4f37634194e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1638,6 +1638,13 @@ static int vmscan_swappiness(struct scan_control *sc)
         return mem_cgroup_swappiness(sc->target_mem_cgroup);
 }
 
+enum scan_balance {
+        SCAN_EQUAL,
+        SCAN_FRACT,
+        SCAN_ANON,
+        SCAN_FILE,
+};
+
 /*
  * Determine how aggressively the anon and file LRU lists should be
  * scanned.  The relative value of each set of LRU lists is determined
@@ -1650,14 +1657,16 @@ static int vmscan_swappiness(struct scan_control *sc)
 static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
                            unsigned long *nr)
 {
-        unsigned long anon, file, free;
+        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
+        u64 fraction[2];
+        u64 denominator = 0;    /* gcc */
+        struct zone *zone = lruvec_zone(lruvec);
         unsigned long anon_prio, file_prio;
+        enum scan_balance scan_balance;
+        unsigned long anon, file, free;
+        bool force_scan = false;
         unsigned long ap, fp;
-        struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
-        u64 fraction[2], denominator;
         enum lru_list lru;
-        bool force_scan = false;
-        struct zone *zone = lruvec_zone(lruvec);
 
         /*
          * If the zone or memcg is small, nr[l] can be 0.  This
@@ -1676,9 +1685,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 
         /* If we have no swap space, do not bother scanning anon pages. */
         if (!sc->may_swap || (nr_swap_pages <= 0)) {
-                fraction[0] = 0;
-                fraction[1] = 1;
-                denominator = 1;
+                scan_balance = SCAN_FILE;
                 goto out;
         }
 
@@ -1690,9 +1697,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
          * too expensive.
          */
         if (!global_reclaim(sc) && !vmscan_swappiness(sc)) {
-                fraction[0] = 0;
-                fraction[1] = 1;
-                denominator = 1;
+                scan_balance = SCAN_FILE;
                 goto out;
         }
 
@@ -1702,9 +1707,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
          * (unless the swappiness setting disagrees with swapping).
          */
         if (!sc->priority && vmscan_swappiness(sc)) {
-                fraction[0] = 1;
-                fraction[1] = 1;
-                denominator = 1;
+                scan_balance = SCAN_EQUAL;
                 goto out;
         }
 
@@ -1722,9 +1725,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
         if (global_reclaim(sc)) {
                 free = zone_page_state(zone, NR_FREE_PAGES);
                 if (unlikely(file + free <= high_wmark_pages(zone))) {
-                        fraction[0] = 1;
-                        fraction[1] = 0;
-                        denominator = 1;
+                        scan_balance = SCAN_ANON;
                         goto out;
                 }
         }
@@ -1734,12 +1735,12 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
          * anything from the anonymous working set right now.
          */
         if (!inactive_file_is_low(lruvec)) {
-                fraction[0] = 0;
-                fraction[1] = 1;
-                denominator = 1;
+                scan_balance = SCAN_FILE;
                 goto out;
         }
 
+        scan_balance = SCAN_FRACT;
+
         /*
          * With swappiness at 100, anonymous and file have the same priority.
          * This scanning priority is essentially the inverse of IO cost.
@@ -1792,9 +1793,31 @@ out:
 
                 size = get_lru_size(lruvec, lru);
                 scan = size >> sc->priority;
+
                 if (!scan && force_scan)
                         scan = min(size, SWAP_CLUSTER_MAX);
-                scan = div64_u64(scan * fraction[file], denominator);
+
+                switch (scan_balance) {
+                case SCAN_EQUAL:
+                        /* Scan lists relative to size */
+                        break;
+                case SCAN_FRACT:
+                        /*
+                         * Scan types proportional to swappiness and
+                         * their relative recent reclaim efficiency.
+                         */
+                        scan = div64_u64(scan * fraction[file], denominator);
+                        break;
+                case SCAN_FILE:
+                case SCAN_ANON:
+                        /* Scan one type exclusively */
+                        if ((scan_balance == SCAN_FILE) != file)
+                                scan = 0;
+                        break;
+                default:
+                        /* Look ma, no brain */
+                        BUG();
+                }
                 nr[lru] = scan;
         }
 }