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([2a01:4b00:bd21:4f00:7cc6:d3ca:494:116c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48323c12d74sm38492195e9.2.2026.02.06.09.57.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Feb 2026 09:57:12 -0800 (PST) Message-ID: Date: Fri, 6 Feb 2026 17:57:14 +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> <20260205235647.GA4177530@nvidia.com> <3281a845-a1b8-468c-a528-b9f6003cddea@gmail.com> <20260206152041.GA1874040@nvidia.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20260206152041.GA1874040@nvidia.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/6/26 15:20, Jason Gunthorpe wrote: > On Fri, Feb 06, 2026 at 03:08:25PM +0000, Pavel Begunkov wrote: >> On 2/5/26 23:56, Jason Gunthorpe wrote: >>> On Thu, Feb 05, 2026 at 07:06:03PM +0000, Pavel Begunkov wrote: >>>> 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. >>> >>> Ok.. but what does that have to do with a user space visible file? >> >> If you're referring to registration taking a file, it's used to forward >> this registration to the right driver, which knows about devices and can >> create dma-buf attachment[s]. The abstraction users get is not just a >> buffer but rather a buffer registered for a "subsystem" represented by >> the passed file. With nvme raw bdev as the only importer in the patch set, >> it's simply converges to "registered for the file", but the notion will >> need to be expanded later, e.g. to accommodate filesystems. > > Sounds completely goofy to me. Hmm... the discussion is not going to be productive, isn't it? > A wrapper around DMABUF that lets you > attach to DMABUFs? Huh? I have no idea what you mean and what "attach to DMABUFs" is. dma-buf is passed to the driver, which attaches it (as in calls dma_buf_dynamic_attach()). > I feel like io uring should be dealing with this internally somehow not > creating more and more uapi.. uapi changes are already minimal and outside of the IO path. > The longer term goal has been to get page * out of the io stack and > start using phys_addr_t, if we could pass the DMABUF's MMIO as a Except that I already tried passing device mapped addresses directly, and it was rejected because it won't be able to handle more complicated cases like multi-device filesystems and probably for other reasons. Or would it be mapping it for each IO? > phys_addr_t around the IO stack then we only need to close the gap of > getting the p2p provider into the final DMA mapping. > > Alot of this has improved in the past few cycles where the main issue > now is the carrying the provider and phys_addr_t through the io to the > nvme driver. vs when you started this and even that fundamental > infrastructure was missing. > >>>>>> 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. >>> >>> I'm not sure what this means? >> >> I'm saying that when a user app can easily get or already has a >> dma-buf fd, it should be easier to just use it instead of finding >> its way to FOLL_PCI_P2PDMA. > > But that all exists already and this proposal does nothing to improve > it.. dma-buf already exists as well, and I'm ashamed to admit, but I don't know how a user program can read into / write from memory provided by dma-buf. >> I'm actually curious, is there a way to somehow create a >> MEMORY_DEVICE_PCI_P2PDMA mapping out of a random dma-buf? > > No. The driver owning the P2P MMIO has to do this during its probe and > then it has to provide a VMA with normal pages so GUP works. This is > usally not hard on the exporting driver side. > > It costs some memory but then everything works naturally in the IO > stack. > > Your project is interesting and would be a nice improvement, but I > also don't entirely understand why you are bothering when the P2PDMA > solution is already fully there ready to go... Is something preventing > you from creating the P2PDMA pages for your exporting driver? I'm not doing it for any particular driver but rather trying to reuse what's already there, i.e. a good coverage of existing dma-buf exporters, and infrastructure dma-buf provides, e.g. move_notify. And trying to do that efficiently, avoiding GUP (what io_uring can already do for normal memory), keeping long term mappings (modulo move_notify), and so. That includes optimising the cost of system memory rw with iommu. -- Pavel Begunkov