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[195.135.220.15]) by mx.google.com with ESMTPS id e26si8366608edq.401.2019.06.23.22.46.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 23 Jun 2019 22:46:54 -0700 (PDT) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id C8667AD76; Mon, 24 Jun 2019 05:46:53 +0000 (UTC) Subject: Re: [PATCH] mm: fix setting the high and low watermarks To: Bharath Vedartham Cc: Alan Jenkins , Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20190621114325.711-1-alan.christopher.jenkins@gmail.com> <3d15b808-b7cd-7379-a6a9-d3cf04b7dcec@suse.cz> <20190621140717.GA28387@bharath12345-Inspiron-5559> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: <20190621140717.GA28387@bharath12345-Inspiron-5559> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 6/21/19 4:07 PM, Bharath Vedartham wrote: > Do you think this could cause a race condition between > __setup_per_zone_wmarks and pgdat_watermark_boosted which checks whether > the watermark_boost of each zone is non-zero? pgdat_watermark_boosted is > not called with a zone lock. > Here is a probable case scenario: > watermarks are boosted in steal_suitable_fallback(which happens under a > zone lock). After that kswapd is woken up by > wakeup_kswapd(zone,0,0,zone_idx(zone)) in rmqueue without holding a > zone lock. Lets say someone modified min_kfree_bytes, this would lead to > all the zone->watermark_boost being set to 0. This may cause > pgdat_watermark_boosted to return false, which would not wakeup kswapd > as intended by boosting the watermark. This behaviour is similar to waking up kswapd for a > balanced node. Not waking up kswapd shouldn't cause a significant trouble. > Also if kswapd was woken up successfully because of watermarks being > boosted. In balance_pgdat, we use nr_boost_reclaim to count number of > pages to reclaim because of boosting. nr_boost_reclaim is calculated as: > nr_boost_reclaim = 0; > for (i = 0; i <= classzone_idx; i++) { > zone = pgdat->node_zones + i; > if (!managed_zone(zone)) > continue; > > nr_boost_reclaim += zone->watermark_boost; > zone_boosts[i] = zone->watermark_boost; > } > boosted = nr_boost_reclaim; > > This is not under a zone_lock. This could lead to nr_boost_reclaim to > be 0 if min_kfree_bytes is set to 0. Which would wake up kcompactd > without reclaiming memory. Setting min_kfree_bytes to 0 is asking for problems regardless of this check. Much more trouble than waking up kcompactd spuriously, which is just a few wasted cpu cycles. > kcompactd compaction might be spurious if the if the memory reclaim step is not happening? > > Any thoughts? Unless the races cause either some data corruption, or e.g. spurious allocation failures, I don't think they are worth adding new spinlock sections. Thanks, Vlastimil >> spin_unlock_irqrestore(&zone->lock, flags); >> >