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Date:   Tue, 5 Jul 2022 12:16:45 -0600
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To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Christoph Hellwig <hch@lst.de>, Jason Gunthorpe <jgg@ziepe.ca>,
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From:   Logan Gunthorpe <logang@deltatee.com>
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Subject: Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Precedence: bulk


On 2022-07-05 11:42, Greg Kroah-Hartman wrote:
> On Tue, Jul 05, 2022 at 11:32:23AM -0600, Logan Gunthorpe wrote:
>>
>>
>> On 2022-07-05 11:21, Greg Kroah-Hartman wrote:
>>> On Tue, Jul 05, 2022 at 06:50:39PM +0200, Christoph Hellwig wrote:
>>>> [note for the newcomers, this is about allowing mmap()ing the PCIe
>>>> P2P memory from the generic PCI P2P code through sysfs, and more
>>>> importantly how to revoke it on device removal]
>>>
>>> We allow mmap on PCIe config space today, right?  Why is this different
>>> from what pci_create_legacy_files() does today?
>>>
>>>> On Tue, Jul 05, 2022 at 10:44:49AM -0600, Logan Gunthorpe wrote:
>>>>> We might be able to. I'm not sure. I'll have to figure out how to find
>>>>> that inode from the p2pdma code. I haven't found an obvious interface to
>>>>> do that.
>>>>
>>>> I think the right way to approach this would be a new sysfs API
>>>> that internally calls unmap_mapping_range internally instead of
>>>> exposing the inode. I suspect that might actually be the right thing
>>>> to do for iomem_inode as well.
>>>
>>> Why do we need something new and how is this any different from the PCI
>>> binary files I mention above?  We have supported PCI hotplug for a very
>>> long time, do the current PCI binary sysfs files not work properly with
>>> mmap and removing a device?
>>
>> The P2PDMA code allocates and hands out struct pages to userspace that
>> are backed with ZONE_DEVICE memory from a device's BAR. This is quite
>> different from the existing binary files mentioned above which neither
>> support struct pages nor allocation.
> 
> Why would you want to do this through a sysfs interface?  that feels
> horrid...

The current version does it through a char device, but that requires
creating a simple_fs and anon_inode for teardown on driver removal, plus
a bunch of hooks through the driver that exposes it (NVMe, in this case)
to set this all up.

Christoph is suggesting a sysfs interface which could potentially avoid
the anon_inode and all of the extra hooks. It has some significant
benefits and maybe some small downsides, but I wouldn't describe it as
horrid.

Logan