linux-kernel - Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap

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Date:   Fri, 6 Nov 2020 15:30:24 -0400
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>,
        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 14/15] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

On Fri, Nov 06, 2020 at 11:20:05AM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2020-11-06 11:09 a.m., Jason Gunthorpe wrote:
> >> Ah, hmm, yes. I guess the pages have to be hooked and returned to the
> >> genalloc through free_devmap_managed_page(). 
> > 
> > That sounds about right, but in this case it doesn't need the VMA
> > operations.
> > 
> >> Seems like it might be doable... but it will complicate things for
> >> users that don't want to use the genpool (though no such users exist
> >> upstream).
> > 
> > I would like to use this stuff in RDMA pretty much immediately and the
> > genpool is harmful for those cases, so please don't make decisions
> > that are tying thing to genpool
> 
> I certainly can't make decisions for code that isn't currently
> upstream.

The rdma drivers are all upstream, what are you thinking about?

> Ultimately, if you aren't using the genpool you will have to implement
> your own mmap operation that somehow allocates the pages and your own
> page_free hook. 

Sure, the mlx5 driver already has a specialized alloctor for it's BAR
pages.

> I also don't expect this to be going upstream in the near term so don't
> get too excited about using it.

I don't know, it is actually not that horrible, the GUP and IOMMU
related changes are simpler than I expected

Jason