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Date:   Tue, 14 Jan 2020 01:41:55 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Oscar Salvador <osalvador@suse.de>
Subject: Re: [PATCH v1 2/2] mm: factor out next_present_section_nr()
Message-ID: <20200113224155.efoekgw4hyey2by2@box>
References: <20200113144035.10848-1-david@redhat.com>
 <20200113144035.10848-3-david@redhat.com>
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On Mon, Jan 13, 2020 at 03:40:35PM +0100, David Hildenbrand wrote:
> Let's move it to the header and use the shorter variant from
> mm/page_alloc.c (the original one will also check
> "__highest_present_section_nr + 1", which is not necessary). While at it,
> make the section_nr in next_pfn() const.
> 
> In next_pfn(), we now return section_nr_to_pfn(-1) instead of -1 once
> we exceed __highest_present_section_nr, which doesn't make a difference in
> the caller as it is big enough (>= all sane end_pfn).
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/mmzone.h | 10 ++++++++++
>  mm/page_alloc.c        | 11 ++---------
>  mm/sparse.c            | 10 ----------
>  3 files changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index c2bc309d1634..462f6873905a 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1379,6 +1379,16 @@ static inline int pfn_present(unsigned long pfn)
>  	return present_section(__nr_to_section(pfn_to_section_nr(pfn)));
>  }
>  
> +static inline unsigned long next_present_section_nr(unsigned long section_nr)
> +{
> +	while (++section_nr <= __highest_present_section_nr) {
> +		if (present_section_nr(section_nr))
> +			return section_nr;
> +	}
> +
> +	return -1;
> +}
> +
>  /*
>   * These are _only_ used during initialisation, therefore they
>   * can use __initdata ...  They could have names to indicate
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a92791512077..26e8044e9848 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5852,18 +5852,11 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
>  /* Skip PFNs that belong to non-present sections */
>  static inline __meminit unsigned long next_pfn(unsigned long pfn)
>  {
> -	unsigned long section_nr;
> +	const unsigned long section_nr = pfn_to_section_nr(++pfn);
>  
> -	section_nr = pfn_to_section_nr(++pfn);
>  	if (present_section_nr(section_nr))
>  		return pfn;
> -
> -	while (++section_nr <= __highest_present_section_nr) {
> -		if (present_section_nr(section_nr))
> -			return section_nr_to_pfn(section_nr);
> -	}
> -
> -	return -1;
> +	return section_nr_to_pfn(next_present_section_nr(section_nr));

This changes behaviour in the corner case: if next_present_section_nr()
returns -1, we call section_nr_to_pfn() for it. It's unlikely would give
any valid pfn, but I can't say for sure for all archs. I guess the worst
case scenrio would be endless loop over the same secitons/pfns.

Have you considered the case?

-- 
 Kirill A. Shutemov