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

[Jason Gunthorpe]

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