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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1rFZjG-002z7C-LZ; Tue, 19 Dec 2023 13:01:46 +0000 Date: Tue, 19 Dec 2023 13:01:46 +0000 From: Matthew Wilcox To: Jingbo Xu Cc: shr@devkernel.io, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, joseph.qi@linux.alibaba.com, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org Subject: Re: [PATCH v2 2/2] mm: fix arithmetic for max_prop_frac when setting max_ratio Message-ID: References: <20231219024246.65654-1-jefflexu@linux.alibaba.com> <20231219024246.65654-3-jefflexu@linux.alibaba.com> <5460aaf1-44f6-475f-b980-cb9058cc1df4@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5460aaf1-44f6-475f-b980-cb9058cc1df4@linux.alibaba.com> X-Rspamd-Queue-Id: 02C5410001C X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: zpakzkx8d1bmi7mgwrp9g4xri51eb4pb X-HE-Tag: 1702990916-512836 X-HE-Meta: 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 t0C/MbNt F8VJ3SmH14YY3GhoJX6ThWVVT5+sUF5gtVPhYv3R7P4tlQ2oaZXasNHxElJj7nj8x+ZBz74SrQRzqHaHwULOwnJdSZwOfjDZex6XVDXbcZdWtN+ofVAiUQumz4I4wyhh1BIeLkWVnsZ/yV2L39wLBm17nU+4FujkFwyYAmIhSRjbEFzZMF2pey1aMFA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.028182, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Dec 19, 2023 at 01:58:21PM +0800, Jingbo Xu wrote: > On 12/19/23 12:06 PM, Matthew Wilcox wrote: > > On Tue, Dec 19, 2023 at 10:42:46AM +0800, Jingbo Xu wrote: > >> } 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, > >> + 100 * BDI_RATIO_SCALE); > >> } > > > > Why use div64_u64 here? > > > > FPROP_FRAC_BASE is an unsigned long. max_ratio is an unsigned int, so > > the numerator is an unsigned long. BDI_RATIO_SCALE is 10,000, so the > > numerator is an unsigned int. There's no 64-bit arithmetic needed here. > > Yes, div64_u64() is actually not needed here. So it seems > > bdi->max_prop_frac = FPROP_FRAC_BASE * max_ratio / 100 / BDI_RATIO_SCALE; > > is adequate? I'd rather spell that as: bdi->max_prop_frac = (FPROP_FRAC_BASE * max_ratio) / (100 * BDI_RATIO_SCALE); It's closer to how you'd write it out mathematically and so it reads more easily. At least for me.