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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48feab2a23dsm102576065e9.5.2026.05.19.02.21.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2026 02:21:50 -0700 (PDT) Date: Tue, 19 May 2026 10:21:48 +0100 From: David Laight To: Pavel Begunkov Cc: Christoph Hellwig , Jens Axboe , Keith Busch , Sagi Grimberg , Alexander Viro , Christian Brauner , Andrew Morton , Sumit Semwal , Christian =?UTF-8?B?S8O2bmln?= , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, Nitesh Shetty , Kanchan Joshi , Anuj Gupta , Tushar Gohad , William Power , Phil Cayton , Jason Gunthorpe Subject: Re: [PATCH v3 04/10] block: introduce dma map backed bio type Message-ID: <20260519102148.21d42afc@pumpkin> In-Reply-To: <24833f76-2289-4859-86d1-9215b11a1258@gmail.com> References: <646ecd6fde8d9e146cb051efb514deb27ce3883e.1777475843.git.asml.silence@gmail.com> <20260513081929.GD5477@lst.de> <24833f76-2289-4859-86d1-9215b11a1258@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 18 May 2026 11:29:54 +0100 Pavel Begunkov wrote: > On 5/13/26 09:19, Christoph Hellwig wrote: > >> + if (!bio_flagged(bio_src, BIO_DMABUF_MAP)) { > >> + bio->bi_io_vec = bio_src->bi_io_vec; > >> + } else { > >> + bio->dmabuf_map = bio_src->dmabuf_map; > >> + bio_set_flag(bio, BIO_DMABUF_MAP); > >> + } > > > > This is backwards, please avoid pointless negations: > > I can flip it, but compilers tend to prefer the true branch. E.g. this > > if (cond) A; else B; > C; > > can get compiled into: > > jmpcc cond B > A: ... > C: > return; > B: ... > jmp C; I'm pretty sure gcc completely ignores the order. Some very old compilers didn't - to the extreme of generating a short conditional branch around a long unconditional branch to get past a large 'true' code block. likely() and unlikely() can change things, they are definitely useful comments to a human (or Human) reader. I'm not sure you can rely on the branch predictor to do anything sensible outside of a loop. -- David