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

David Hildenbrand
2025-02-26T20:06:08Z

On 26.02.25 20:54, Alex Williamson wrote:
> 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?

I assume it's fine. Andrew is on CC, so he should be aware of it. I'm
not aware of possible clashes.

--
Cheers,

David / dhildenb