On Thu, Mar 11, 2021 at 11:35:24AM +0000, Christoph Hellwig wrote: > On Wed, Mar 10, 2021 at 03:06:07PM -0500, Peter Xu wrote: > > On Wed, Mar 10, 2021 at 02:40:11PM -0400, Jason Gunthorpe wrote: > > > On Wed, Mar 10, 2021 at 11:34:06AM -0700, Alex Williamson wrote: > > > > > > > > I think after the address_space changes this should try to stick with > > > > > a normal io_rmap_pfn_range() done outside the fault handler. > > > > > > > > I assume you're suggesting calling io_remap_pfn_range() when device > > > > memory is enabled, > > > > > > Yes, I think I saw Peter thinking along these lines too > > > > > > Then fault just always causes SIGBUS if it gets called > > I feel much more comfortable having the io_remap_pfn_range in place. It's just that Jason convinced me with the fact that io_remap_pfn_range() will modify vma flags, and I tend to agree that's not a good thing to do during a fault() handler (in remap_pfn_range): vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP; Although this case is special and it does not do harm it seems, since all these four flags are already set by vfio_pci_mmap() anyways, so the flag didn't really change at least with current code base. It's just still cleaner to not use io_remap_pfn_range() in vfio fault() since future change to the function io_remap_pfn_range() may not guarantee to match with vfio mmap(). > > > > > Indeed that looks better than looping in the fault(). > > > > But I don't know whether it'll be easy to move io_remap_pfn_range() to device > > memory enablement. If it's a two-step thing, we can fix the BUG_ON and vma > > duplication issue first, then the full rework can be done in the bigger series > > as what be chosen as the last approach. > > What kind of problems do you envision? It seems pretty simple to do, > at least when combined with the unmap_mapping_range patch. Moving the prefault into device memory enablement will even remove the 1st fault delay when doing the first MMIO access that triggers this fault(). Also in that case I think we can also call io_remap_pfn_range() directly and safely, rather than looping over vmf_insert_pfn_prot(). Thanks, -- Peter Xu
