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[217.140.101.70]) by mx.google.com with ESMTP id z23si41601eji.328.2019.02.13.05.21.03 for ; Wed, 13 Feb 2019 05:21:04 -0800 (PST) Received-SPF: pass (google.com: domain of will.deacon@arm.com designates 217.140.101.70 as permitted sender) client-ip=217.140.101.70; Authentication-Results: mx.google.com; spf=pass (google.com: domain of will.deacon@arm.com designates 217.140.101.70 as permitted sender) smtp.mailfrom=will.deacon@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 134E480D; Wed, 13 Feb 2019 05:21:03 -0800 (PST) Received: from brain-police (usa-sjc-mx-foss1.foss.arm.com [217.140.101.70]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 59A333F557; Wed, 13 Feb 2019 05:21:00 -0800 (PST) Date: Wed, 13 Feb 2019 13:20:57 +0000 From: Will Deacon To: Mel Gorman Cc: Andrew Morton , Yury Norov , Vlastimil Babka , Andrea Arcangeli , David Rientjes , Michal Hocko , Catalin Marinas , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org Subject: Re: [PATCH] mm, page_alloc: Fix a division by zero error when boosting watermarks Message-ID: <20190213132056.GE5875@brain-police> References: <20190213131923.GQ9565@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190213131923.GQ9565@techsingularity.net> User-Agent: Mutt/1.9.4 (2018-02-28) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Feb 13, 2019 at 01:19:23PM +0000, Mel Gorman wrote: > Yury Norov reported that an arm64 KVM instance could not boot since after > v5.0-rc1 and could addressed by reverting the patches > > 1c30844d2dfe272d58c ("mm: reclaim small amounts of memory when an external > 73444bc4d8f92e46a20 ("mm, page_alloc: do not wake kswapd with zone lock held") > > The problem is that a division by zero error is possible if boosting occurs > either very early in boot or if the high watermark is very small. This > patch checks for the conditions and avoids boosting in those cases. > > Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs") > Reported-and-tested-by: Yury Norov > Signed-off-by: Mel Gorman > --- > mm/page_alloc.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index d295c9bc01a8..ae7e4ba5b9f5 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2170,6 +2170,11 @@ static inline void boost_watermark(struct zone *zone) > > max_boost = mult_frac(zone->_watermark[WMARK_HIGH], > watermark_boost_factor, 10000); > + > + /* high watermark be be uninitialised or very small */ > + if (!max_boost) > + return; > + > max_boost = max(pageblock_nr_pages, max_boost); > > zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages, I can confirm that this also allows my KVM guest to boot: Tested-by: Will Deacon Will