[PATCH] mm: vmscan: correct nr_reclaimed for THP

From: Yang Shi
Date: Thu May 09 2019 - 20:17:58 EST

Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
swapped out"), THP can be swapped out in a whole. But, nr_reclaimed
still gets inc'ed by one even though a whole THP (512 pages) gets
swapped out.

This doesn't make too much sense to memory reclaim. For example, direct
reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
could fulfill it. But, if nr_reclaimed is not increased correctly,
direct reclaim may just waste time to reclaim more pages,
SWAP_CLUSTER_MAX * 512 pages in worst case.

This change may result in more reclaimed pages than scanned pages showed
by /proc/vmstat since scanning one head page would reclaim 512 base pages.

Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
I'm not quite sure if it was the intended behavior or just omission. I tried
to dig into the review history, but didn't find any clue. I may miss some

mm/vmscan.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fd9de50..7e026ec 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1446,7 +1446,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,

- nr_reclaimed++;
+ /*
+ * THP may get swapped out in a whole, need account
+ * all base pages.
+ */
+ nr_reclaimed += (1 << compound_order(page));

* Is there need to periodically free_page_list? It would