linux-kernel - Re: [PATCH v2 5/6] mm: Provide address mask in struct follow_pfnmap

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Message-ID: <3d1315ab-ba94-46c2-8dbf-ef26454f7007@redhat.com>
Date: Wed, 19 Feb 2025 09:31:48 +0100
From: David Hildenbrand <david@...hat.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterx@...hat.com,
 mitchell.augustin@...onical.com, clg@...hat.com, jgg@...dia.com,
 akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 5/6] mm: Provide address mask in struct
 follow_pfnmap_args

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@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: linux-mm@...ck.org
> Reviewed-by: Peter Xu <peterx@...hat.com>
> Reviewed-by: "Mitchell Augustin" <mitchell.augustin@...onical.com>
> Tested-by: "Mitchell Augustin" <mitchell.augustin@...onical.com>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb