linux-kernel - Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA segments in an SGL

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Date:   Wed, 9 Dec 2020 17:22:09 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "open list:DMA MAPPING HELPERS" <iommu@...ts.linux-foundation.org>,
        Stephen Bates <sbates@...thlin.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC PATCH 04/15] lib/scatterlist: Add flag for indicating P2PDMA
 segments in an SGL

On Mon, Nov 9, 2020 at 8:47 AM Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
>
> On 2020-11-09 2:12 a.m., Christoph Hellwig wrote:
> > On Fri, Nov 06, 2020 at 10:00:25AM -0700, Logan Gunthorpe wrote:
> >> We make use of the top bit of the dma_length to indicate a P2PDMA
> >> segment.
> >
> > I don't think "we" can.  There is nothing limiting the size of a SGL
> > segment.
>
> Yes, I expected this would be the unacceptable part. Any alternative ideas?

Why is the SG_P2PDMA_FLAG needed as compared to checking the SGL
segment-pages for is_pci_p2pdma_page()?