[109/131] mm: vmscan: drop unnecessary div0 avoidance rounding in get_scan_count()

Commit Message

Andrew Morton June 3, 2020, 11:02 p.m. UTC

From: Johannes Weiner <[e-mail]>
Subject: mm: vmscan: drop unnecessary div0 avoidance rounding in get_scan_count()

When we calculate the relative scan pressure between the anon and file LRU
lists, we have to assume that reclaim_stat can contain zeroes.  To avoid
div0 crashes, we add 1 to all denominators like so:

        anon_prio = swappiness;
        file_prio = 200 - anon_prio;

	[...]

        /*
         * The amount of pressure on anon vs file pages is inversely
         * proportional to the fraction of recently scanned pages on
         * each list that were recently referenced and in active use.
         */
        ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
        ap /= reclaim_stat->recent_rotated[0] + 1;

        fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
        fp /= reclaim_stat->recent_rotated[1] + 1;
        spin_unlock_irq(&pgdat->lru_lock);

        fraction[0] = ap;
        fraction[1] = fp;
        denominator = ap + fp + 1;

While reclaim_stat can contain 0, it's not actually possible for ap + fp
to be 0.  One of anon_prio or file_prio could be zero, but they must still
add up to 200.  And the reclaim_stat fraction, due to the +1 in there, is
always at least 1.  So if one of the two numerators is 0, the other one
can't be.  ap + fp is always at least 1.  Drop the + 1.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Johannes Weiner <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/vmscan.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

--- a/mm/vmscan.c~mm-vmscan-drop-unnecessary-div0-avoidance-rounding-in-get_scan_count
+++ a/mm/vmscan.c
@@ -2348,7 +2348,7 @@  static void get_scan_count(struct lruvec
 
 	fraction[0] = ap;
 	fraction[1] = fp;
-	denominator = ap + fp + 1;
+	denominator = ap + fp;
 out:
 	for_each_evictable_lru(lru) {
 		int file = is_file_lru(lru);