linux-kernel - Re: [PATCH v3 4/20] PCI/P2PDMA: introduce helpers for dma_map

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Message-ID: <91469404-fd20-effa-2e01-aa79d9d4b9b5@deltatee.com>
Date:   Wed, 29 Sep 2021 15:30:42 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>
Subject: Re: [PATCH v3 4/20] PCI/P2PDMA: introduce helpers for dma_map_sg
 implementations




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