linux-kernel - Re: [PATCH v1 2/2] mm: factor out next_present_section

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Date:   Mon, 13 Jan 2020 17:57:12 -0500 (EST)
From:   David Hildenbrand <dhildenb@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1 2/2] mm: factor out next_present_section_nr()



> Am 13.01.2020 um 23:41 schrieb Kirill A. Shutemov <kirill@...temov.name>:
> 
> 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@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...nel.org>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Kirill A. Shutemov <kirill@...temov.name>
>> Signed-off-by: David Hildenbrand <david@...hat.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?

Yes, see the patch description. We return -1 << PFN_SECTION_SHIFT, so a number close to the end of the address space (0xfff...000). (Will double check tomorrow if any 32bit arch could be problematic here)

Thanks!

> 
> -- 
> Kirill A. Shutemov
>