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 19:48:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.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 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting
 PCI P2PDMA pages

On Wed, Sep 29, 2021 at 03:34:22PM -0600, Logan Gunthorpe wrote:
> 
> 
> 
> 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.

It would be done under the pte devmap test and this is less expensive
than the xarray search.

Jason