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 Some of this seems out of date.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
