Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Ira Weiny <iweiny@intel.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>
References: <20201106170036.18713-1-logang@deltatee.com>
 <20201106170036.18713-15-logang@deltatee.com>
 <20201106172206.GS36674@ziepe.ca>
 <b1e8dfce-d583-bed8-d04d-b7265a54c99f@deltatee.com>
 <20201106174223.GU36674@ziepe.ca>
 <2c2d2815-165e-2ef9-60d6-3ace7ff3aaa5@deltatee.com>
 <20201106180922.GV36674@ziepe.ca>
 <09885400-36f8-bc1d-27f0-a8adcf6104d4@deltatee.com>
 <20201106193024.GW36674@ziepe.ca>
 <03032637-0826-da76-aec2-121902b1c166@deltatee.com>
 <20201106195341.GA244516@ziepe.ca>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <e6a99d54-b33a-0df1-ee33-4aa7a70124a6@deltatee.com>
Date:   Fri, 6 Nov 2020 13:03:26 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.4.0
MIME-Version: 1.0
In-Reply-To: <20201106195341.GA244516@ziepe.ca>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Subject: Re: [RFC PATCH 14/15] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Precedence: bulk


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