linux-kernel - Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap

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Date:   Tue, 5 Jul 2022 13:29:59 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christoph Hellwig <hch@....de>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        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>,
        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>,
        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>,
        Ralph Campbell <rcampbell@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

On Tue, Jul 05, 2022 at 06:12:40PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 05, 2022 at 10:51:02AM -0300, Jason Gunthorpe wrote:
> > > In fact I'm not even sure this should be a character device, it seems
> > > to fit it way better with the PCI sysfs hierchacy, just like how we
> > > map MMIO resources, which these are anyway.  And once it is on sysfs
> > > we do have a uniqueue inode and need none of the pseudofs stuff, and
> > > don't need all the glue code in nvme either.
> > 
> > Shouldn't there be an allocator here? It feels a bit weird that the
> > entire CMB is given to a single process, it is a sharable resource,
> > isn't it?
> 
> Making the entire area given by the device to the p2p allocator available
> to user space seems sensible to me.  That is what the current series does,
> and what a sysfs interface would do as well.

That makes openning the mmap exclusive with the in-kernel allocator -
so it means opening the mmap fails if something else is using a P2P
page and once the mmap is open all kernel side P2P allocations will
fail?

Which seems inelegant, I would expect the the mmap operation to
request some pages from the P2P allocator and provide them to
userspace so user and kernel workflows can co-exist using the same
CMB.

Jason