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

David Hildenbrand
2025-02-19T08:31:59Z

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>

--
Cheers,

David / dhildenb