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

[Logan Gunthorpe <logang@deltatee.com>]

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.

> This is very inflexable, it would be better if this is designed like
> io_remap_pfn where it just preps and fills the VMA, doesn't take
> ownership of the entire VMA

If someone wants to manage their own P2P memory and create their own
userspace mapping with vmf_insert_mixed they are free to do so. But this
helper is specifically for users of pci_p2pdma_map_alloc(). I know you
don't intend to use that but it doesn't make it any less valid.

Logan