Re: [PATCH v3 19/20] PCI/P2PDMA: introduce pci_mmap_p2pmem()

[Jason Gunthorpe <jgg@ziepe.ca>]

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.

> 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

Jason