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[89.243.246.11]) by smtp.gmail.com with ESMTPSA id o2sm1698861wrq.56.2019.06.21.04.44.05 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Fri, 21 Jun 2019 04:44:06 -0700 (PDT) From: Alan Jenkins To: Mel Gorman Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Alan Jenkins , stable@vger.kernel.org Subject: [PATCH] mm: fix setting the high and low watermarks Date: Fri, 21 Jun 2019 12:43:25 +0100 Message-Id: <20190621114325.711-1-alan.christopher.jenkins@gmail.com> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When setting the low and high watermarks we use min_wmark_pages(zone). I guess this is to reduce the line length. But we forgot that this macro includes zone->watermark_boost. We need to reset zone->watermark_boost first. Otherwise the watermarks will be set inconsistently. E.g. this could cause inconsistent values if the watermarks have been boosted, and then you change a sysctl which triggers __setup_per_zone_wmarks(). I strongly suspect this explains why I have seen slightly high watermarks. Suspicious-looking zoneinfo below - notice high-low != low-min. Node 0, zone Normal pages free 74597 min 9582 low 34505 high 36900 https://unix.stackexchange.com/questions/525674/my-low-and-high-watermarks-seem-higher-than-predicted-by-documentation-sysctl-vm/525687 Signed-off-by: Alan Jenkins Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") Cc: stable@vger.kernel.org --- Tested by compiler :-). Ideally the commit message would be clear about what happens the *first* time __setup_per_zone_watermarks() is called. I guess that zone->watermark_boost is *usually* zero, or we would have noticed some wild problems :-). However I am not familiar with how the zone structures are allocated & initialized. Maybe there is a case where zone->watermark_boost could contain an arbitrary unitialized value at this point. Can we rule that out? mm/page_alloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index c02cff1ed56e..db9758cda6f8 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -7606,9 +7606,9 @@ static void __setup_per_zone_wmarks(void) mult_frac(zone_managed_pages(zone), watermark_scale_factor, 10000)); + zone->watermark_boost = 0; zone->_watermark[WMARK_LOW] = min_wmark_pages(zone) + tmp; zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2; - zone->watermark_boost = 0; spin_unlock_irqrestore(&zone->lock, flags); } -- 2.20.1