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

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Date:   Tue, 14 Jan 2020 00:02:00 +0100
From:   David Hildenbrand <david@...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 23:57 schrieb David Hildenbrand <dhildenb@...hat.com>:
> 
> 
> 
>>> 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)

... but thinking again, 0xfff... is certainly an invalid PFN, so this should work just fine.

(biggest possible pfn is -1 >> PFN_SHIFT)

But it‘s late in Germany, will double check tomorrow :)