[RFC] vmscan: Scale file_is_tiny calculation based on priority

[Nico Pache <npache@redhat.com>]

Now that reclaiming anon memory is more prevelant (Johannes describes this
well in commit f53af4285d77 ("mm: vmscan: fix extreme overreclaim and swap
floods")), we've been seeing large bursts (sometimes in the order of
multiple GiBs) of anon memory being reclaimed despite swappiness being
very low (=1) and there being plenty of page cache remaining.

Johannes commit f53af4285d77 ("mm: vmscan: fix extreme overreclaim and
swap floods"), helped reduce these swap storms; however, it did not fully
curb this effect. Under further investigation I noticed these swap
storms correspond to the activation of file_is_tiny.

file_is_tiny is being computed on a per-node basis, if reclaim
drains the page cache on one node, and the scheduler is prefering
new allocations on a separate node, file_is_tiny will remain elevated
for a very long time, constantly draining anon from the node that is low
on page cache.

These burst of reclaim are also seen in the single node case, where once
file_is_tiny=1, anon reclaim is too aggressive with a low swap value.

Reduce these extreme bursts of anon reclaim by scaling the total_high_wmark
down by the reclaim priority. This will activate file_is_tiny way less
often, and for smaller bursts.

Fixes: ccc5dc67340c ("mm/vmscan: make active/inactive ratio as 1:1 for anon lru")
Fixes: 5df741963d52 ("mm: fix LRU balancing effect of new transparent huge pages")
Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 026199c047e0..0d288bb5354e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2882,7 +2882,7 @@ static void prepare_scan_count(pg_data_t *pgdat, struct scan_control *sc)
 		anon = node_page_state(pgdat, NR_INACTIVE_ANON);
 
 		sc->file_is_tiny =
-			file + free <= total_high_wmark &&
+			file + free <= (total_high_wmark >> sc->priority) &&
 			!(sc->may_deactivate & DEACTIVATE_ANON) &&
 			anon >> sc->priority;
 	}
-- 
2.38.1