mm: avoid swapping out with swappiness==0

commit fe35004fbf9eaf67482b074a2e032abb9c89b1dd upstream.

Sometimes we'd like to avoid swapping out anonymous memory.  In
particular, avoid swapping out pages of important process or process
groups while there is a reasonable amount of pagecache on RAM so that we
can satisfy our customers' requirements.

OTOH, we can control how aggressive the kernel will swap memory pages with
/proc/sys/vm/swappiness for global and
/sys/fs/cgroup/memory/memory.swappiness for each memcg.

But with current reclaim implementation, the kernel may swap out even if
we set swappiness=0 and there is pagecache in RAM.

This patch changes the behavior with swappiness==0.  If we set
swappiness==0, the kernel does not swap out completely (for global reclaim
until the amount of free pages and filebacked pages in a zone has been
reduced to something very very small (nr_free + nr_filebacked < high
watermark)).

Signed-off-by: Satoru Moriya <satoru.moriya@hds.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
 - Adjust context
 - vmscan_swappiness() does not have a zone parameter]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 48febd7..86eb848 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1977,10 +1977,10 @@ static void get_scan_count(struct zone *zone, struct scan_control *sc,
         * proportional to the fraction of recently scanned pages on
         * each list that were recently referenced and in active use.
         */
-       ap = (anon_prio + 1) * (reclaim_stat->recent_scanned[0] + 1);
+       ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
        ap /= reclaim_stat->recent_rotated[0] + 1;
 
-       fp = (file_prio + 1) * (reclaim_stat->recent_scanned[1] + 1);
+       fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
        fp /= reclaim_stat->recent_rotated[1] + 1;
        spin_unlock_irq(&zone->lru_lock);
 
@@ -1993,7 +1993,7 @@ out:
                unsigned long scan;
 
                scan = zone_nr_lru_pages(zone, sc, l);
-               if (priority || noswap) {
+               if (priority || noswap || !vmscan_swappiness(sc)) {
                        scan >>= priority;
                        if (!scan && force_scan)
                                scan = SWAP_CLUSTER_MAX;