Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     linux-mm@kvack.org
Cc:     Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan.kim@gmail.com>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: [PATCH 07/14] mm: vmscan: drop unnecessary div0 avoidance rounding in get_scan_count()
Date:   Wed, 20 May 2020 19:25:18 -0400
Message-Id: <20200520232525.798933-8-hannes@cmpxchg.org>
In-Reply-To: <20200520232525.798933-1-hannes@cmpxchg.org>
References: <20200520232525.798933-1-hannes@cmpxchg.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When we calculate the relative scan pressure between the anon and file
LRU lists, we have to assume that reclaim_stat can contain zeroes. To
avoid div0 crashes, we add 1 to all denominators like so:

        anon_prio = swappiness;
        file_prio = 200 - anon_prio;

	[...]

        /*
         * The amount of pressure on anon vs file pages is inversely
         * proportional to the fraction of recently scanned pages on
         * each list that were recently referenced and in active use.
         */
        ap = anon_prio * (reclaim_stat->recent_scanned[0] + 1);
        ap /= reclaim_stat->recent_rotated[0] + 1;

        fp = file_prio * (reclaim_stat->recent_scanned[1] + 1);
        fp /= reclaim_stat->recent_rotated[1] + 1;
        spin_unlock_irq(&pgdat->lru_lock);

        fraction[0] = ap;
        fraction[1] = fp;
        denominator = ap + fp + 1;

While reclaim_stat can contain 0, it's not actually possible for ap +
fp to be 0. One of anon_prio or file_prio could be zero, but they must
still add up to 200. And the reclaim_stat fraction, due to the +1 in
there, is always at least 1. So if one of the two numerators is 0, the
other one can't be. ap + fp is always at least 1. Drop the + 1.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/vmscan.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 43f88b1a4f14..6cd1029ea9d4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2348,7 +2348,7 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 
 	fraction[0] = ap;
 	fraction[1] = fp;
-	denominator = ap + fp + 1;
+	denominator = ap + fp;
 out:
 	for_each_evictable_lru(lru) {
 		int file = is_file_lru(lru);
-- 
2.26.2