linux-kernel - Re: [PATCH v3 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting PCI P2PDMA pages

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Date:   Wed, 29 Sep 2021 15:34:22 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
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 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting
 PCI P2PDMA pages




On 2021-09-28 1:47 p.m., Jason Gunthorpe wrote:
> On Thu, Sep 16, 2021 at 05:40:54PM -0600, Logan Gunthorpe wrote:
>> Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
>> allow obtaining P2PDMA pages. If a caller does not set this flag
>> and tries to map P2PDMA pages it will fail.
>>
>> This is implemented by adding a flag and a check to get_dev_pagemap().
> 
> I would like to see the get_dev_pagemap() deleted from GUP in the
> first place.
> 
> Why isn't this just a simple check of the page->pgmap type after
> acquiring a valid page reference? See my prior note

It could be, but that will mean dereferencing the pgmap for every page
to determine the type of page and then comparing with FOLL_PCI_P2PDMA.

Probably not terrible to go this way.

Logan