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From:   Yang Shi <yang.shi@linux.alibaba.com>
To:     ying.huang@intel.com, hannes@cmpxchg.org, mhocko@suse.com,
        mgorman@techsingularity.net, kirill.shutemov@linux.intel.com,
        hughd@google.com, akpm@linux-foundation.org
Cc:     yang.shi@linux.alibaba.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH] mm: vmscan: correct nr_reclaimed for THP
Date:   Fri, 10 May 2019 08:16:32 +0800
Message-Id: <1557447392-61607-1-git-send-email-yang.shi@linux.alibaba.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Yang Shi <yang.shi@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