[PATCH] mm/vmscan: adjust shrinkctl->nr_scanned after invoking scan_objects

[Wei Wang]

Some shrinkers may free more than the requested nr_to_scan of pages
in one invocation of scan_objects, and some may free less than that.

Currently shrinkers can either return the actual number of pages that
have been freed via the return value of scan_objects or track that
actual number in shrinkctl->nr_scanned. But do_shrink_slab works on an
assumption that the actual number is always tracked via
shrinkctl->nr_scanned, which is not true. Having checked the shrinkers
used in the kernel, they basically return the actual number of freed
pages via the return value of scan_objects, and most of them leave
shrinkctl->nr_scanned unchanged after scan_objects is called.

So this patch adjusts shrinkctl->nr_scanned to the actual freed number
after scan_objects is called.

Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
 mm/vmscan.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 03822f8..78a75b9 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -447,9 +447,13 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 		if (ret == SHRINK_STOP)
 			break;
 		freed += ret;
+		shrinkctl->nr_scanned = ret;
 
 		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
-		total_scan -= shrinkctl->nr_scanned;
+		if (total_scan > shrinkctl->nr_scanned)
+			total_scan -= shrinkctl->nr_scanned;
+		else
+			total_scan = 0;
 		scanned += shrinkctl->nr_scanned;
 
 		cond_resched();
-- 
2.7.4