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 20:04:00 -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 Wed, Dec 9, 2020 at 6:07 PM Logan Gunthorpe <logang@...tatee.com> wrote:
>
>
>
> On 2020-12-09 6:22 p.m., Dan Williams wrote:
> > 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()?
>
> Because the DMA and page segments in the SGL aren't necessarily aligned...
>
> The IOMMU implementations can coalesce multiple pages into fewer DMA
> address ranges, so the page pointed to by sg->page_link may not be the
> one that corresponds to the address in sg->dma_address for a given segment.
>
> If that makes sense -- it's not the easiest thing to explain.

It does...

Did someone already grab, or did you already consider the 3rd
available bit in page_link? AFAICS only SG_CHAIN and SG_END are
reserved. However, if you have a CONFIG_64BIT dependency for
user-directed p2pdma that would seem to allow SG_P2PDMA_FLAG to be
(0x4) in page_link.