[patch 2/2] vmscan: clip swap_cluster_max in shrink_all_memory()

shrink_inactive_list() scans in sc->swap_cluster_max chunks until it
hits the scan limit it was passed.

shrink_inactive_list()
{
	do {
		isolate_pages(swap_cluster_max)
		shrink_page_list()
	} while (nr_scanned < max_scan);
}

This assumes that swap_cluster_max is not bigger than the scan limit
because the latter is checked only after at least one iteration.

In shrink_all_memory() sc->swap_cluster_max is initialized to the
overall reclaim goal in the beginning but not decreased while reclaim
is making progress which leads to subsequent calls to
shrink_inactive_list() reclaiming way too much in the one iteration
that is done unconditionally.

Set sc->swap_cluster_max always to the proper goal before doing
  shrink_all_zones()
    shrink_list()
      shrink_inactive_list().

While the current shrink_all_memory() happily reclaims more than
actually requested, this patch fixes it to never exceed the goal:

unpatched
   wanted=10000 reclaimed=13356
   wanted=10000 reclaimed=19711
   wanted=10000 reclaimed=10289
   wanted=10000 reclaimed=17306
   wanted=10000 reclaimed=10700
   wanted=10000 reclaimed=10004
   wanted=10000 reclaimed=13301
   wanted=10000 reclaimed=10976
   wanted=10000 reclaimed=10605
   wanted=10000 reclaimed=10088
   wanted=10000 reclaimed=15000

patched
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=9599
   wanted=10000 reclaimed=8476
   wanted=10000 reclaimed=8326
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=9919
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=9624
   wanted=10000 reclaimed=10000
   wanted=10000 reclaimed=10000
   wanted=8500 reclaimed=8092
   wanted=316 reclaimed=316

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: KOSAKI Motohiro <redacted>
Cc: MinChan Kim <redacted>
Cc: "Rafael J. Wysocki" <redacted>
Cc: Nigel Cunningham <redacted>
Cc: Rik van Riel <redacted>
---
 mm/vmscan.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -2105,7 +2105,6 @@ unsigned long shrink_all_memory(unsigned
 	struct scan_control sc = {
 		.gfp_mask = GFP_KERNEL,
 		.may_unmap = 0,
-		.swap_cluster_max = nr_pages,
 		.may_writepage = 1,
 		.isolate_pages = isolate_pages_global,
 	};

@@ -2147,6 +2146,7 @@ unsigned long shrink_all_memory(unsigned
 			unsigned long nr_to_scan = nr_pages - sc.nr_reclaimed;
 
 			sc.nr_scanned = 0;
+			sc.swap_cluster_max = nr_to_scan;
 			shrink_all_zones(nr_to_scan, prio, pass, &sc);
 			if (sc.nr_reclaimed >= nr_pages)
 				goto out;


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