Re: [PATCH v2 5/6] mm: Provide address mask in struct follow_pfnmap_args

Alex Williamson
2025-02-26T19:54:45Z

On Wed, 19 Feb 2025 09:31:48 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 18.02.25 23:22, 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 address mask of the page table level is already provided to this
> > latter function for calculating the pfn. This address 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 address 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@linux-foundation.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: linux-mm@kvack.org
> > Reviewed-by: Peter Xu <peterx@redhat.com>
> > Reviewed-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
> > Tested-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
> > Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks, David!

Is there any objection from mm folks to bring this in through the vfio
tree?

Patch: https://lore.kernel.org/all/20250218222209.1382449-6-alex.williamson@redhat.com/
Series: https://lore.kernel.org/all/20250218222209.1382449-1-alex.williamson@redhat.com/

Thanks,
Alex