mm: vmscan: improve comment on low-page cache handling

Fix comment style and elaborate on why anonymous memory is force-scanned
when file cache runs low.

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, 7 insertions, 5 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 68586c887611..259f8208a388 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1713,13 +1713,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
         file  = get_lru_size(lruvec, LRU_ACTIVE_FILE) +
                 get_lru_size(lruvec, LRU_INACTIVE_FILE);
 
+        /*
+         * If it's foreseeable that reclaiming the file cache won't be
+         * enough to get the zone back into a desirable shape, we have
+         * to swap.  Better start now and leave the - probably heavily
+         * thrashing - remaining file pages alone.
+         */
         if (global_reclaim(sc)) {
-                free  = zone_page_state(zone, NR_FREE_PAGES);
+                free = zone_page_state(zone, NR_FREE_PAGES);
                 if (unlikely(file + free <= high_wmark_pages(zone))) {
-                        /*
-                         * If we have very few page cache pages, force-scan
-                         * anon pages.
-                         */
                         fraction[0] = 1;
                         fraction[1] = 0;
                         denominator = 1;