linux-kernel - Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap

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Date:   Wed, 29 Sep 2021 20:05:43 -0300
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>,
        John Hubbard <jhubbard@...dia.com>,
        Don Dutile <ddutile@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Jakowski Andrzej <andrzej.jakowski@...el.com>,
        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>
Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

On Wed, Sep 29, 2021 at 03:42:00PM -0600, Logan Gunthorpe wrote:

> The main reason is probably this: if we don't use VM_MIXEDMAP, then we
> can't set pte_devmap(). 

I think that is an API limitation in the fault routines..

finish_fault() should set the pte_devmap - eg by passing the
PFN_DEV|PFN_MAP somehow through the vma->vm_page_prot to mk_pte() or
otherwise signaling do_set_pte() that it should set those PTE bits
when it creates the entry.

(or there should be a vmf_* helper for this special case, but using
the vmf->page seems righter to me)

> If we don't set pte_devmap(), then every single page that GUP
> processes needs to check if it's a ZONE_DEVICE page and also if it's
> a P2PDMA page (thus dereferencing pgmap) in order to satisfy the
> requirements of FOLL_PCI_P2PDMA.

Definately not suggesting not to set pte_devmap(), only that
VM_MIXEDMAP should not be set on VMAs that only contain struct
pages. That is an abuse of what it is intended for.

At the very least there should be a big comment above the usage
explaining that this is just working around a limitation in
finish_fault() where it cannot set the PFN_DEV|PFN_MAP bits today.

Jason