Hi, I'm interested in extending remap_pfn_range to allow it to map the range hugely (using PUDs or PMDs). The initial user I have in mind is vfio-pci; I'm thinking when we're mapping large ranges for GPUs, we can get both a performance and host overhead win by doing this hugely. Another thing I have in the back of my mind is adding something KVM can re-use to simplify its whole host_pfn_mapping_level / hva_to_pfn_remapped / get_user_page_fast_only thing. I know Peter and David are working on some related things (hugetlbfs unification and follow_pte et al improvements, respectively). Although I have a hacky proof of concept that works, I thought it best to get some consensus on the design before I post something, so I don't conflict with this existing / upcoming work. Changing remap_pfn_range to install PUDs or PMDs is straightforward. The hairy part is the fault / follow side of things: 1. follow_pte clearly doesn't work for this, since the leaf might be a PUD or PMD instead. Most callers don't care about the PTE itself, they care about the pgprot or flags it has set, so my idea was to add a new interface which just yields those bits, instead of the actual PTE. Peter, I think hugetlbfs unification may run into similar issues, do you have some plan already to deal with PUD/PMD/PTE being different types? 2. vfio-pci relies on vm_ops->fault. This is a problem because the normal fault handler path doesn't call this until after it has walked down to the PTE level, installing PUDs/PMDs along the way. I have only gross ideas for how to deal with this: - Add a VM_HUGEPFNMAP VMA flag indicating vm_ops->fault should be called earlier in __handle_mm_fault - Add a vm_ops->hugepfn_fault (name not important) which should be called earlier in __handle_mm_fault - Go ahead and let remap_pfn_range overwrite existing PUDs/PMDS I wonder which of these folks find least offensive? Or is there a better way I haven't thought of? 3. That's also an issue for CoW faults, but I don't know of any real use case for CoW huge pfn mappings, so I thought we can just keep the existing small mapping behavior for CoW VMAs. Any objections? Thanks!
