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

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Message-ID: <20201106174223.GU36674@ziepe.ca>
Date:   Fri, 6 Nov 2020 13:42:23 -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 10:28:00AM -0700, Logan Gunthorpe wrote:
> 
> 
> On 2020-11-06 10:22 a.m., Jason Gunthorpe wrote:
> > On Fri, Nov 06, 2020 at 10:00:35AM -0700, Logan Gunthorpe wrote:
> >> Introduce pci_mmap_p2pmem() which is a helper to allocate and mmap
> >> a hunk of p2pmem into userspace.
> >>
> >> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> >>  drivers/pci/p2pdma.c       | 104 +++++++++++++++++++++++++++++++++++++
> >>  include/linux/pci-p2pdma.h |   6 +++
> >>  2 files changed, 110 insertions(+)
> >>
> >> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> >> index 9961e779f430..8eab53ac59ae 100644
> >> +++ b/drivers/pci/p2pdma.c
> >> @@ -16,6 +16,7 @@
> >>  #include <linux/genalloc.h>
> >>  #include <linux/memremap.h>
> >>  #include <linux/percpu-refcount.h>
> >> +#include <linux/pfn_t.h>
> >>  #include <linux/random.h>
> >>  #include <linux/seq_buf.h>
> >>  #include <linux/xarray.h>
> >> @@ -1055,3 +1056,106 @@ ssize_t pci_p2pdma_enable_show(char *page, struct pci_dev *p2p_dev,
> >>  	return sprintf(page, "%s\n", pci_name(p2p_dev));
> >>  }
> >>  EXPORT_SYMBOL_GPL(pci_p2pdma_enable_show);
> >> +
> >> +struct pci_p2pdma_map {
> >> +	struct kref ref;
> >> +	struct pci_dev *pdev;
> >> +	void *kaddr;
> >> +	size_t len;
> >> +};
> > 
> > Why have this at all? Nothing uses it and no vm_operations ops are
> > implemented?
> 
> It's necessary to free the allocated p2pmem when the mapping is torn down.

That's suspicious.. Once in a VMA the lifetime of the page must be
controlled by the page refcount, it can't be put back into the genpool
just because the vma was destroed.

Jason