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

[Logan Gunthorpe]

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@xxxxxxxxxxxx>
>>>>  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