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Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <20210928220502.GA1738588@nvidia.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-CA Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 24.64.144.200 X-SA-Exim-Rcpt-To: ckulkarnilinux@gmail.com, martin.oliveira@eideticom.com, robin.murphy@arm.com, ira.weiny@intel.com, helgaas@kernel.org, jianxin.xiong@intel.com, dave.hansen@linux.intel.com, jason@jlekstrand.net, dave.b.minturn@intel.com, andrzej.jakowski@intel.com, daniel.vetter@ffwll.ch, willy@infradead.org, ddutile@redhat.com, jhubbard@nvidia.com, christian.koenig@amd.com, dan.j.williams@intel.com, hch@lst.de, sbates@raithlin.com, iommu@lists.linux-foundation.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, linux-block@vger.kernel.org, linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org, jgg@nvidia.com X-SA-Exim-Mail-From: logang@deltatee.com Subject: Re: [PATCH v3 4/20] PCI/P2PDMA: introduce helpers for dma_map_sg implementations X-SA-Exim-Version: 4.2.1 (built Wed, 08 May 2019 21:11:16 +0000) X-SA-Exim-Scanned: Yes (on ale.deltatee.com) Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=deltatee.com header.s=20200525 header.b=G14JPhTM; spf=pass (imf26.hostedemail.com: domain of logang@deltatee.com designates 204.191.154.188 as permitted sender) smtp.mailfrom=logang@deltatee.com; dmarc=pass (policy=none) header.from=deltatee.com X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 1EECC20019FD X-Stat-Signature: i74xukp7yg7e5phgu6ma9zomsucuj1bu X-HE-Tag: 1632951063-151067 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2021-09-28 4:05 p.m., Jason Gunthorpe wrote: > On Thu, Sep 16, 2021 at 05:40:44PM -0600, Logan Gunthorpe wrote: > >> +enum pci_p2pdma_map_type >> +pci_p2pdma_map_segment(struct pci_p2pdma_map_state *state, struct device *dev, >> + struct scatterlist *sg) >> +{ >> + if (state->pgmap != sg_page(sg)->pgmap) { >> + state->pgmap = sg_page(sg)->pgmap; > > This has built into it an assumption that every page in the sg element > has the same pgmap, but AFAIK nothing enforces this rule? There is no > requirement that the HW has pfn gaps between the pgmaps linux decides > to create over it. No, that's not a correct reading of the code. Every time there is a new pagemap, this code calculates the mapping type and bus offset. If a page comes along with a different page map,f it recalculates. This just reduces the overhead so that the calculation is done only every time a page with a different pgmap comes along and not doing it for every single page. > At least sg_alloc_append_table_from_pages() and probably something in > the block world should be updated to not combine struct pages with > different pgmaps, and this should be documented in scatterlist.* > someplace. There's no sane place to do this check. The code is designed to support mappings with different pgmaps. Logan