linux-kernel - Re: [PATCH 4/5] mm: Provide page mask in struct follow_pfnmap

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Message-ID: <20250214101735.4b180123.alex.williamson@redhat.com>
Date: Fri, 14 Feb 2025 10:17:35 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: alex.williamson@...hat.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterx@...hat.com,
 mitchell.augustin@...onical.com, clg@...hat.com, akpm@...ux-foundation.org,
 linux-mm@...ck.org, David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 4/5] mm: Provide page mask in struct follow_pfnmap_args


Nudge.  Peter Xu provided an R-b for the series.  Would any other mm
folks like to chime in here to provide objection or approval for this
change and merging it through the vfio tree?  Series[1].  Thanks!

Alex

[1]https://lore.kernel.org/all/20250205231728.2527186-1-alex.williamson@redhat.com/

On Wed,  5 Feb 2025 16:17:20 -0700
Alex Williamson <alex.williamson@...hat.com> 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@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  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;