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Tue, 3 Jun 2025 15:55:26 +0000 Message-ID: Date: Tue, 3 Jun 2025 17:55:18 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range To: Christoph Hellwig Cc: wangtao , sumit.semwal@linaro.org, kraxel@redhat.com, vivek.kasireddy@intel.com, viro@zeniv.linux.org.uk, brauner@kernel.org, hughd@google.com, akpm@linux-foundation.org, amir73il@gmail.com, benjamin.gaignard@collabora.com, Brian.Starkey@arm.com, jstultz@google.com, tjmercier@google.com, jack@suse.cz, baolin.wang@linux.alibaba.com, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, bintian.wang@honor.com, yipengxiang@honor.com, liulu.liu@honor.com, feng.han@honor.com References: <20250603095245.17478-1-tao.wangtao@honor.com> <09c8fb7c-a337-4813-9f44-3a538c4ee8b1@amd.com> <924ac01f-b86b-4a03-b563-878fa7736712@amd.com> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: Content-Type: text/plain; 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Why does the page table lookup tage so much > time compared to normal I/O? I have absolutely no idea. It's rather surprising for me as well. The user seems to have a rather slow CPU paired with fast I/O, but it still looks rather fishy to me. Additional to that allocating memory through memfd_create() is *much* slower on that box than through dma-buf-heaps (which basically just uses GFP and an array). We have seen something similar with customers systems which we couldn't explain so far. >> My question is rather if it's ok to call f_op->write_iter() and >> f_op->read_iter() with pages allocated by alloc_pages(), e.g. >> where drivers potentially ignore the page count and just re-use pages >> as they like? > > read_iter and write_iter with ITER_BVEC just use the pages as source > and destination of the I/O. They must not touch the refcounts or > do anything fancy with them. Various places in the kernel rely on > that. Perfect, thanks for that info. Regards, Christian.