linux-kernel - Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap

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Message-ID: <b3deacdd-cb76-6ebb-0e29-ef6a5a426a0d@deltatee.com>
Date:   Tue, 5 Jul 2022 12:16:45 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Christoph Hellwig <hch@....de>, Jason Gunthorpe <jgg@...pe.ca>,
        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>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Minturn Dave B <dave.b.minturn@...el.com>,
        Jason Ekstrand <jason@...kstrand.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xiong Jianxin <jianxin.xiong@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Robin Murphy <robin.murphy@....com>,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@...il.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()



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