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
