linux-kernel - Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

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Message-ID: <CAPcyv4hCNoMeFyOE588=kuNUXaPS-rzaXnF2cN2TFejso1SGRw@mail.gmail.com>
Date:   Wed, 26 Jun 2019 13:39:01 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-pci@...r.kernel.org, linux-rdma <linux-rdma@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <kbusch@...nel.org>,
        Stephen Bates <sbates@...thlin.com>
Subject: Re: [RFC PATCH 00/28] Removing struct page from P2PDMA

On Wed, Jun 26, 2019 at 1:21 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Wed, Jun 26, 2019 at 12:31:08PM -0600, Logan Gunthorpe wrote:
> > > we have a hole behind len where we could store flag.  Preferably
> > > optionally based on a P2P or other magic memory types config
> > > option so that 32-bit systems with 32-bit phys_addr_t actually
> > > benefit from the smaller and better packing structure.
> >
> > That seems sensible. The one thing that's unclear though is how to get
> > the PCI Bus address when appropriate. Can we pass that in instead of the
> > phys_addr with an appropriate flag? Or will we need to pass the actual
> > physical address and then, at the map step, the driver has to some how
> > lookup the PCI device to figure out the bus offset?
>
> I agree with CH, if we go down this path it is a layering violation
> for the thing injecting bio's into the block stack to know what struct
> device they egress&dma map on just to be able to do the dma_map up
> front.
>
> So we must be able to go from this new phys_addr_t&flags to some BAR
> information during dma_map.
>
> For instance we could use a small hash table of the upper phys addr
> bits, or an interval tree, to do the lookup.

Hmm, that sounds like dev_pagemap without the pages.

There's already no requirement that dev_pagemap point to real /
present pages (DEVICE_PRIVATE) seems a straightforward extension to
use it for helping coordinate phys_addr_t in 'struct bio'. Then
Logan's future plans to let userspace coordinate p2p operations could
build on PTE_DEVMAP.