Re: [RFC PATCH 1/3] mm: Introduce for_each_valid_pfn() and use it from reserve_bootmem_region()

[Mike Rapoport <rppt@kernel.org>]

On Wed, Apr 02, 2025 at 09:18:39PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> Especially since commit 9092d4f7a1f8 ("memblock: update initialization
> of reserved pages"), the reserve_bootmem_region() function can spend a
> significant amount of time iterating over every 4KiB PFN in a range,
> calling pfn_valid() on each one, and ultimately doing absolutely nothing.
> 
> On a platform used for virtualization, with large NOMAP regions that
> eventually get used for guest RAM, this leads to a significant increase
> in steal time experienced during kexec for a live update.
> 
> Introduce for_each_valid_pfn() and use it from reserve_bootmem_region().
> This implementation is precisely the same naïve loop that the function
> used to have, but subsequent commits will provide optimised versions
> for FLATMEM and SPARSEMEM, and this version will remain for those
> architectures which provide their own pfn_valid() implementation,
> until/unless they also provide a matching for_each_valid_pfn().
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  include/linux/mmzone.h | 10 ++++++++++
>  mm/mm_init.c           | 23 ++++++++++-------------
>  2 files changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 25e80b2ca7f4..32ecb5cadbaf 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2176,6 +2176,16 @@ void sparse_init(void);
>  #define subsection_map_init(_pfn, _nr_pages) do {} while (0)
>  #endif /* CONFIG_SPARSEMEM */
>  
> +/*
> + * Fallback case for when the architecture provides its own pfn_valid() but
> + * not a corresponding for_each_valid_pfn().
> + */
> +#ifndef for_each_valid_pfn
> +#define for_each_valid_pfn(_pfn, _start_pfn, _end_pfn)			\
> +	for ((_pfn) = (_start_pfn); (_pfn) < (_end_pfn); (_pfn)++)	\
> +		if (pfn_valid(_pfn))
> +#endif
> +
>  #endif /* !__GENERATING_BOUNDS.H */
>  #endif /* !__ASSEMBLY__ */
>  #endif /* _LINUX_MMZONE_H */
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index a38a1909b407..7c699bad42ad 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -777,22 +777,19 @@ static inline void init_deferred_page(unsigned long pfn, int nid)
>  void __meminit reserve_bootmem_region(phys_addr_t start,
>  				      phys_addr_t end, int nid)
>  {
> -	unsigned long start_pfn = PFN_DOWN(start);
> -	unsigned long end_pfn = PFN_UP(end);
> +	unsigned long pfn;
>  
> -	for (; start_pfn < end_pfn; start_pfn++) {
> -		if (pfn_valid(start_pfn)) {
> -			struct page *page = pfn_to_page(start_pfn);
> +	for_each_valid_pfn (pfn, PFN_DOWN(start), PFN_UP(end)) {
> +		struct page *page = pfn_to_page(pfn);
>  
> -			init_deferred_page(start_pfn, nid);
> +		init_deferred_page(pfn, nid);
>  
> -			/*
> -			 * no need for atomic set_bit because the struct
> -			 * page is not visible yet so nobody should
> -			 * access it yet.
> -			 */
> -			__SetPageReserved(page);
> -		}
> +		/*
> +		 * no need for atomic set_bit because the struct
> +		 * page is not visible yet so nobody should
> +		 * access it yet.
> +		 */
> +		__SetPageReserved(page);
>  	}
>  }
>  
> -- 
> 2.49.0
> 

-- 
Sincerely yours,
Mike.