Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap_p2pmem()

From: Logan Gunthorpe <logang@deltatee.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
	linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-mm@kvack.org, iommu@lists.linux-foundation.org,
	"Stephen Bates" <sbates@raithlin.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Christian König" <christian.koenig@amd.com>,
	"Ira Weiny" <iweiny@intel.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	"Don Dutile" <ddutile@redhat.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>
Subject: Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Date: Fri, 6 Nov 2020 10:53:45 -0700	[thread overview]
Message-ID: <2c2d2815-165e-2ef9-60d6-3ace7ff3aaa5@deltatee.com> (raw)
In-Reply-To: <20201106174223.GU36674@ziepe.ca>

On 2020-11-06 10:42 a.m., Jason Gunthorpe wrote:
> 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@deltatee.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.

Ah, hmm, yes. I guess the pages have to be hooked and returned to the
genalloc through free_devmap_managed_page(). 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).

Logan