linux-kernel - Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap

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Date:   Fri, 6 Nov 2020 13:03:26 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     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>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Christian König <christian.koenig@....com>,
        Ira Weiny <iweiny@...el.com>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap_p2pmem()




On 2020-11-06 12:53 p.m., Jason Gunthorpe wrote:
> On Fri, Nov 06, 2020 at 12:44:59PM -0700, Logan Gunthorpe wrote:
>>
>>
>> On 2020-11-06 12:30 p.m., Jason Gunthorpe wrote:
>>>> I certainly can't make decisions for code that isn't currently
>>>> upstream.
>>>
>>> The rdma drivers are all upstream, what are you thinking about?
>>
>> Really? I feel like you should know what I mean here...
>>
>> I mean upstream code that actually uses the APIs that I'd have to
>> introduce. I can't say here's an API feature that no code uses but the
>> already upstream rdma driver might use eventually. It's fairly easy to
>> send patches that make the necessary changes when someone adds a use of
>> those changes inside the rdma code.
> 
> Sure, but that doesn't mean you have to actively design things to be
> unusable beyond this narrow case. The RDMA drivers are all there, all
> upstream, if this work is accepted then the changes to insert P2P
> pages into their existing mmap flows is a reasonable usecase to
> consider at this point when building core code APIs.
> 
> You shouldn't add dead code, but at least have a design in mind for
> what it needs to look like and some allowance.

I don't see anything I've done that's at odds with that. You will still
need to make changes to the p2pdma code to implement your use case.

>>>> Ultimately, if you aren't using the genpool you will have to implement
>>>> your own mmap operation that somehow allocates the pages and your own
>>>> page_free hook. 
>>>
>>> Sure, the mlx5 driver already has a specialized alloctor for it's BAR
>>> pages.
>>
>> So it *might* make sense to carve out a common helper to setup a VMA for
>> P2PDMA to do the vm_flags check and set VM_MIXEDMAP... but besides that,
>> there's no code that would be common to the two cases.
> 
> I think the whole insertion of P2PDMA pages into a VMA should be
> similar to io_remap_pfn() so all the VM flags, pgprot_decrypted and
> other subtle details are all in one place. (also it needs a
> pgprot_decrypted before doing vmf_insert, I just learned that this
> month)

I don't think a function like that will work for the p2pmem use case. In
order to implement proper page freeing I expect I'll need a loop around
the allocator and vm_insert_mixed()... Something roughly like:

for (addr = vma->vm_start; addr < vma->vm_end; addr += PAGE_SIZE) {
        vaddr = pci_alloc_p2pmem(pdev, PAGE_SIZE);
	ret = vmf_insert_mixed(vma, addr,
  		       __pfn_to_pfn_t(virt_to_pfn(vaddr), PFN_DEV | PFN_MAP));
}

That way we can call pci_free_p2pmem() when a page's ref count goes to
zero. I suspect your use case will need to do something similar.

Logan