On Wed, Feb 05, 2025 at 04:17:20PM -0700, Alex Williamson wrote: > follow_pfnmap_start() walks the page table for a given address and > fills out the struct follow_pfnmap_args in pfnmap_args_setup(). > The page mask of the page table level is already provided to this > latter function for calculating the pfn. This page mask can also be > useful for the caller to determine the extent of the contiguous > mapping. > > For example, vfio-pci now supports huge_fault for pfnmaps and is able > to insert pud and pmd mappings. When we DMA map these pfnmaps, ex. > PCI MMIO BARs, we iterate follow_pfnmap_start() to get each pfn to test > for a contiguous pfn range. Providing the mapping page mask allows us > to skip the extent of the mapping level. Assuming a 1GB pud level and > 4KB page size, iterations are reduced by a factor of 256K. In wall > clock time, mapping a 32GB PCI BAR is reduced from ~1s to <1ms. > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: linux-mm@xxxxxxxxx > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx> > --- > include/linux/mm.h | 2 ++ > mm/memory.c | 1 + > 2 files changed, 3 insertions(+) Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> Jason
