linux-kernel - Re: [PATCH v4 1/7] mm: Introduce for_each_valid_pfn() and use it from reserve_bootmem

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Message-ID: <0eae5cc8-5714-44dc-97b4-e1b991c0e918@redhat.com>
Date: Thu, 24 Apr 2025 23:11:43 +0200
From: David Hildenbrand <david@...hat.com>
To: David Woodhouse <dwmw2@...radead.org>, Mike Rapoport <rppt@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Sauerwein, David" <dssauerw@...zon.de>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Ard Biesheuvel <ardb@...nel.org>, Catalin Marinas <catalin.marinas@....com>,
 Marc Zyngier <maz@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mike Rapoport <rppt@...ux.ibm.com>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, Ruihan Li <lrh2000@....edu.cn>
Subject: Re: [PATCH v4 1/7] mm: Introduce for_each_valid_pfn() and use it from
 reserve_bootmem_region()

On 23.04.25 15:33, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.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@...zon.co.uk>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.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 6ccec1bf2896..230a29c2ed1a 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2177,6 +2177,16 @@ 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 9659689b8ace..41884f2155c4 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)) {

                           ^ space should be removed

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

-- 
Cheers,

David / dhildenb