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From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <f9a83402-3d66-7437-ca47-77bac4108424@deltatee.com>
Date:   Wed, 29 Sep 2021 17:49:36 -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 5:35 p.m., Jason Gunthorpe wrote:
> On Wed, Sep 29, 2021 at 05:27:22PM -0600, Logan Gunthorpe wrote:
> 
>>> 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().
> 
> I thought VM_MIXEDMAP was quite clear:
> 
> #define VM_MIXEDMAP	0x10000000	/* Can contain "struct page" and pure PFN pages */
> 
> This VMA does not include PFN pages, so it should not be tagged
> VM_MIXEDMAP.
> 
> Aside from enabling the special vmf_ API, it only controls some
> special behavior in vm_normal_page:
> 
>  * VM_MIXEDMAP mappings can likewise contain memory with or without "struct
>  * page" backing, however the difference is that _all_ pages with a struct
>  * page (that is, those where pfn_valid is true) are refcounted and considered
>  * normal pages by the VM. The disadvantage is that pages are refcounted
>  * (which can be slower and simply not an option for some PFNMAP users). The
>  * advantage is that we don't have to follow the strict linearity rule of
>  * PFNMAP mappings in order to support COWable mappings.
> 
> Which again does not describe this case.

Some of this seems out of date. Pretty sure the pages are not refcounted
with vmf_insert_mixed() and vmf_insert_mixed() is currently the only way
to use VM_MIXEDMAP mappings.

>> 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).
> 
> hacky hacky :)
> 
> I think DAX probably did it that way for the same reason you are
> doing it that way - no other choice without changing something

Sure but if you look at other vmf_insert_mixed() (of which there are
few) you see similar patterns. Seems more like it was documented with
one thing in mind but then used in a completely different manner. Which
is why I suggested the documentation was not so good.

Logan