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

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Message-ID: <20201107001457.GB244516@ziepe.ca>
Date:   Fri, 6 Nov 2020 20:14:57 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Logan Gunthorpe <logang@...tatee.com>
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 Fri, Nov 06, 2020 at 01:03:26PM -0700, Logan Gunthorpe wrote:
> 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.

Yes, but I would say the pci_alloc_p2pmem() layer should be able to
free pages on a page-by-page basis so you don't have to do stuff like
the above.

There is often a lot of value in having physical contiguous addresses,
so allocating page by page as well seems poor.

Jason