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SubjectRe: [PATCH v1 2/2] mm: factor out next_present_section_nr()
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

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