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([2620:10d:c092:600::1:640]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-497fecfccc7sm26554895e9.12.2026.07.29.04.46.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Jul 2026 04:46:07 -0700 (PDT) Message-ID: Date: Wed, 29 Jul 2026 12:45:47 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 09/14] nvme-pci: add SGL support for the dmabuf path To: Christoph Hellwig , Anuj Gupta/Anuj Gupta Cc: Jens Axboe , Keith Busch , Sagi Grimberg , 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, Alexander Viro , Christian Brauner , Andrew Morton , Sumit Semwal , =?UTF-8?Q?Christian_K=C3=B6nig?= , Nitesh Shetty , Kanchan Joshi , Tushar Gohad , William Power , Phil Cayton , Jason Gunthorpe , Damien Le Moal , Alasdair Kergon , Mike Snitzer , Mikulas Patocka , Benjamin Marzinski , Vishal Verma , David Sterba , Ilya Dryomov , dm-devel@lists.linux.dev, nvdimm@lists.linux.dev, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, cpgs@samsung.com References: <5c2e9c3bdd3e87c0c9a6e4395ea199002b83fd36.1785274111.git.asml.silence@gmail.com> <20260729072117.GI9534@lst.de> <81412691.21785320403945.JavaMail.epsvc@epcpadp1new> <20260729113108.GA31120@lst.de> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20260729113108.GA31120@lst.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/29/26 12:31, Christoph Hellwig wrote: > On Wed, Jul 29, 2026 at 03:47:23PM +0530, Anuj Gupta/Anuj Gupta wrote: >>> But I also don't understand what the use case for this function >>> is to start with. struct sg_table tells us how many segments >>> exist on the DMA side in the nents member, which should be just >>> fine for the SGL threshold calculation. >> >> sg_table->nents covers the entire exported buffer (<=1GiB), while a >> request only covers a subrange[bi_offset, bi_offset+payload). Using >> nents would overcount the request's segments. > > Urgg, yes. > >>>> + if (!entries) >>>> + return BLK_STS_IOERR; >>>> + if (entries > NVME_MAX_SEGS) >>>> + return BLK_STS_AGAIN; >>> >>> Given that the block layer enforced data in rw/command and the >>> max_segments limit, why do we need the extra check here? >> >> A dmabuf bio reports nsegs=1 (bio_split_io_at) to the block layer, so >> max_segments isn't enforced against the SG entries actually spanned by >> the request. Hence the explicit check. > > We'll need to expose the actual nsegs to the block layer and split > based on that. Otherwise I/O might work or fail based on the device > capabilities. It was exposed in early version as I was passing a [{dma,len}, ...] array, but we moved from that. Maybe I should put the minimum segment size in the map structure, and (possibly over) split using that for now? Keith had this chunk in his patches: + int offset = offset_in_page(bio->bi_iter.bi_bvec_done); + + nsegs = ALIGN(bio->bi_iter.bi_size + offset, PAGE_SIZE) >> + PAGE_SHIFT; + if (bio->bi_iter.bi_size > max_bytes) { + bytes = max_bytes; + nsegs = (bytes + offset) >> PAGE_SHIFT; + } else if (nsegs > lim->max_segments) { + nsegs = lim->max_segments; + bytes = PAGE_SIZE * nsegs - offset; + } else { + *segs = nsegs; + return NULL; + } -- Pavel Begunkov