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([2a01:4b00:bd21:4f00:7cc6:d3ca:494:116c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483203d5ef7sm2743825e9.1.2026.02.05.11.06.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Feb 2026 11:06:07 -0800 (PST) Message-ID: Date: Thu, 5 Feb 2026 19:06:03 +0000 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [LSF/MM/BPF TOPIC] dmabuf backed read/write To: Jason Gunthorpe Cc: linux-block@vger.kernel.org, io-uring , "linux-nvme@lists.infradead.org" , "Gohad, Tushar" , =?UTF-8?Q?Christian_K=C3=B6nig?= , Christoph Hellwig , Kanchan Joshi , Anuj Gupta , Nitesh Shetty , "lsf-pc@lists.linux-foundation.org" References: <4796d2f7-5300-4884-bd2e-3fcc7fdd7cea@gmail.com> <20260205174135.GA444713@nvidia.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20260205174135.GA444713@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/5/26 17:41, Jason Gunthorpe wrote: > On Tue, Feb 03, 2026 at 02:29:55PM +0000, Pavel Begunkov wrote: > >> The proposal consists of two parts. The first is a small in-kernel >> framework that allows a dma-buf to be registered against a given file >> and returns an object representing a DMA mapping. > > What is this about and why would you need something like this? > > The rest makes more sense - pass a DMABUF (or even memfd) to iouring > and pre-setup the DMA mapping to get dma_addr_t, then directly use > dma_addr_t through the entire block stack right into the eventual > driver. That's more or less what I tried to do in v1, but 1) people didn't like the idea of passing raw dma addresses directly, and having it wrapped into a black box gives more flexibility like potentially supporting multi-device filesystems. And 2) dma-buf folks want dynamic attachments, and it makes it quite a bit more complicated when you might be asked to shoot down DMA mappings at any moment, so I'm isolating all that into something that can be reused. >> Tushar was helping and mention he got good numbers for P2P transfers >> compared to bouncing it via RAM. > > We can already avoid the bouncing, it seems the main improvements here > are avoiding the DMA map per-io and allowing the use of P2P without > also creating struct page. Meanginful wins for sure. Yes, and it should probably be nicer for frameworks that already expose dma-bufs. -- Pavel Begunkov