linux-kernel - Re: [RFC PATCH v2 4/7] mm: Optimise SPARSEMEM implementation of for_each_valid

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Message-ID: <Z_N5ps86xJmewe_P@kernel.org>
Date: Mon, 7 Apr 2025 10:07:18 +0300
From: Mike Rapoport <rppt@...nel.org>
To: David Woodhouse <dwmw2@...radead.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>,
	David Hildenbrand <david@...hat.com>, Marc Zyngier <maz@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mike Rapoport <rppt@...ux.ibm.com>, Will Deacon <will@...nel.org>,
	kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v2 4/7] mm: Optimise SPARSEMEM implementation of
 for_each_valid_pfn()

On Fri, Apr 04, 2025 at 04:59:56PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> There's no point in checking the section and subsection bitmap for *every*
> PFN in the same section; they're either all valid or they aren't.

Don't you want to merge this with the previous commit?
 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  include/linux/mmzone.h | 39 +++++++++++++++++++++++++++++----------
>  1 file changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 67cdf675a4b9..0da1b0ba5d9f 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -2154,21 +2154,20 @@ static inline int pfn_valid(unsigned long pfn)
>  	return ret;
>  }
>  
> -static inline bool first_valid_pfn(unsigned long *p_pfn)
> +/* Returns -1 (an invalid PFN) if no valid PFN remaining */
> +static inline unsigned long first_valid_pfn(unsigned long pfn, unsigned long end_pfn)
>  {
> -	unsigned long pfn = *p_pfn;
>  	unsigned long nr = pfn_to_section_nr(pfn);
>  
>  	rcu_read_lock_sched();
>  
> -	while (nr <= __highest_present_section_nr) {
> +	while (nr <= __highest_present_section_nr && pfn < end_pfn) {
>  		struct mem_section *ms = __pfn_to_section(pfn);
>  
>  		if (valid_section(ms) &&
>  		    (early_section(ms) || pfn_section_first_valid(ms, &pfn))) {
> -			*p_pfn = pfn;
>  			rcu_read_unlock_sched();
> -			return true;
> +			return pfn;
>  		}
>  
>  		/* Nothing left in this section? Skip to next section */
> @@ -2177,14 +2176,34 @@ static inline bool first_valid_pfn(unsigned long *p_pfn)
>  	}
>  
>  	rcu_read_unlock_sched();
> +	return (unsigned long)-1;
> +}
>  
> -	return false;
> +static inline unsigned long next_valid_pfn(unsigned long pfn, unsigned long end_pfn)
> +{
> +	pfn++;
> +
> +	if (pfn >= end_pfn)
> +		return (unsigned long)-1;
> +
> +	/*
> +	 * Either every PFN within the section (or subsection for VMEMMAP) is
> +	 * valid, or none of them are. So there's no point repeating the check
> +	 * for every PFN; only call first_valid_pfn() the first time, and when
> +	 * crossing a (sub)section boundary (i.e. !(pfn & ~PFN_VALID_MASK)).
> +	 */
> +	if (pfn & (IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) ?
> +		   PAGE_SUBSECTION_MASK : PAGE_SECTION_MASK))
> +		return pfn;
> +
> +	return first_valid_pfn(pfn, end_pfn);
>  }
>  
> -#define for_each_valid_pfn(_pfn, _start_pfn, _end_pfn)	       \
> -	for ((_pfn) = (_start_pfn);			       \
> -	     first_valid_pfn(&(_pfn)) && (_pfn) < (_end_pfn);  \
> -	     (_pfn)++)
> +
> +#define for_each_valid_pfn(_pfn, _start_pfn, _end_pfn)			\
> +	for ((_pfn) = first_valid_pfn((_start_pfn), (_end_pfn));	\
> +	     (_pfn) != (unsigned long)-1;				\
> +	     (_pfn) = next_valid_pfn((_pfn), (_end_pfn)))
>  
>  #endif
>  
> -- 
> 2.49.0
> 

-- 
Sincerely yours,
Mike.