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
