From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-98.freemail.mail.aliyun.com (out30-98.freemail.mail.aliyun.com [115.124.30.98]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 62768187F for ; Tue, 19 Dec 2023 02:03:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R171e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045192;MF=jefflexu@linux.alibaba.com;NM=1;PH=DS;RN=5;SR=0;TI=SMTPD_---0VyogQL4_1702951421; Received: from 30.221.145.29(mailfrom:jefflexu@linux.alibaba.com fp:SMTPD_---0VyogQL4_1702951421) by smtp.aliyun-inc.com; Tue, 19 Dec 2023 10:03:42 +0800 Message-ID: <112dd87b-1072-4568-93d5-3b13ed818511@linux.alibaba.com> Date: Tue, 19 Dec 2023 10:03:40 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: fix arithmetic for bdi min_ratio and max_ratio Content-Language: en-US To: Andrew Morton Cc: shr@devkernel.io, linux-mm@kvack.org, linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com References: <20231218031640.77983-1-jefflexu@linux.alibaba.com> <20231218102117.c91ab02353ff76b952f3ec9e@linux-foundation.org> From: Jingbo Xu In-Reply-To: <20231218102117.c91ab02353ff76b952f3ec9e@linux-foundation.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 12/19/23 2:21 AM, Andrew Morton wrote: > On Mon, 18 Dec 2023 11:16:40 +0800 Jingbo Xu wrote: > >> bdi->[min|max]_ratio are both part per million. Fix the wrong >> arithmetic when setting bdi's min_ratio and max_ratio. >> >> Fixes: efc3e6ad53ea ("mm: split off __bdi_set_max_ratio() function") >> Fixes: 8021fb3232f2 ("mm: split off __bdi_set_min_ratio() function") > > As we have two Fixes:, it would be better to have two patches please. Sounds reasonable. Thanks. > >> --- a/mm/page-writeback.c >> +++ b/mm/page-writeback.c >> @@ -692,7 +692,6 @@ static int __bdi_set_min_ratio(struct backing_dev_info *bdi, unsigned int min_ra >> >> if (min_ratio > 100 * BDI_RATIO_SCALE) >> return -EINVAL; >> - min_ratio *= BDI_RATIO_SCALE; This indeed has visible runtime effect as follows: # cat /sys/class/bdi/253\:0/min_ratio 0 # cat /sys/class/bdi/253\:0/max_ratio 100 # echo 1 > /sys/class/bdi/253\:0/min_ratio -bash: echo: write error: Invalid argument Setting min_ratio will fail with -EINVAL, as the above code tries to set min_ratio with (min_ratio * BDI_RATIO_SCALE)%, i.e. 10000% in the above example. >> >> spin_lock_bh(&bdi_lock); >> if (min_ratio > bdi->max_ratio) { >> @@ -729,7 +728,8 @@ static int __bdi_set_max_ratio(struct backing_dev_info *bdi, unsigned int max_ra >> ret = -EINVAL; >> } else { >> bdi->max_ratio = max_ratio; >> - bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / 100; >> + bdi->max_prop_frac = div64_u64(FPROP_FRAC_BASE * max_ratio, >> + 100UL * BDI_RATIO_SCALE); >> } >> spin_unlock_bh(&bdi_lock); This one has no visible runtime effect, but I believe this would affect the incrementing of writeout completion count when max_ratio is not 100%. -- Thanks, Jingbo