Re: [PATCH 4/5] mm: Provide page mask in struct follow_pfnmap_args

[Mitchell Augustin]

Reviewed-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx>
Tested-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx>


On Wed, Feb 5, 2025 at 5:18 PM Alex Williamson
<alex.williamson@xxxxxxxxxx> 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(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index b1c3db9cf355..0ef7e7a0b4eb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2416,11 +2416,13 @@ struct follow_pfnmap_args {
>          * Outputs:
>          *
>          * @pfn: the PFN of the address
> +        * @pgmask: page mask covering pfn
>          * @pgprot: the pgprot_t of the mapping
>          * @writable: whether the mapping is writable
>          * @special: whether the mapping is a special mapping (real PFN maps)
>          */
>         unsigned long pfn;
> +       unsigned long pgmask;
>         pgprot_t pgprot;
>         bool writable;
>         bool special;
> diff --git a/mm/memory.c b/mm/memory.c
> index 398c031be9ba..97ccd43761b2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6388,6 +6388,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args,
>         args->lock = lock;
>         args->ptep = ptep;
>         args->pfn = pfn_base + ((args->address & ~addr_mask) >> PAGE_SHIFT);
> +       args->pgmask = addr_mask;
>         args->pgprot = pgprot;
>         args->writable = writable;
>         args->special = special;
> --
> 2.47.1
>


--
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering