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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <32ce26d7-86e9-f8d5-f0cf-40497946efe9@deltatee.com>
Date:   Wed, 29 Sep 2021 17:27:22 -0600
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Subject: Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()
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On 2021-09-29 5:05 p.m., Jason Gunthorpe wrote:
> 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)

I'm not opposed to this. Though I'm not sure what's best here.

>> 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.

Is it? Documentation on vmf_insert_mixed() and VM_MIXEDMAP is not good
and the intention is not clear. I got the impression that mm people
wanted those interfaces used for users of pte_devmap().

device-dax uses these interfaces and as far as I can see it also only
contains struct pages (or at least  dev_dax_huge_fault() calls
pfn_to_page() on every page when VM_FAULT_NOPAGE happens).

So it would be nice to get some direction here from mm developers on
what they'd prefer.

Logan