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.hu...@intel.com> Cc: Johannes Weiner <han...@cmpxchg.org> Cc: Michal Hocko <mho...@suse.com> Cc: Mel Gorman <mgor...@techsingularity.net> Cc: "Kirill A . Shutemov" <kirill.shute...@linux.intel.com> Cc: Hugh Dickins <hu...@google.com> Signed-off-by: Yang Shi <yang....@linux.alibaba.com> --- 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 discussion. 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, unlock_page(page); free_it: - 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 -- 1.8.3.1