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From:   David Hildenbrand <david@redhat.com>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v1 2/2] mm: factor out next_present_section_nr()
Date:   Tue, 14 Jan 2020 00:02:00 +0100
Message-Id: <C40ACB72-F8C7-4F9B-B3F3-00FBC0C44406@redhat.com>
References: <0B77E39C-BD38-4A61-AB28-3578B519952F@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>, 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>
In-Reply-To: <0B77E39C-BD38-4A61-AB28-3578B519952F@redhat.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> Am 13.01.2020 um 23:57 schrieb David Hildenbrand <dhildenb@redhat.com>:
>=20
> =EF=BB=BF
>=20
>>> Am 13.01.2020 um 23:41 schrieb Kirill A. Shutemov <kirill@shutemov.name>=
:
>>>=20
>>> =EF=BB=BFOn Mon, Jan 13, 2020 at 03:40:35PM +0100, David Hildenbrand wro=
te:
>>> 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.
>>>=20
>>> 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 i=
n
>>> the caller as it is big enough (>=3D all sane end_pfn).
>>>=20
>>> 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(-)
>>>=20
>>> 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)));
>>> }
>>>=20
>>> +static inline unsigned long next_present_section_nr(unsigned long secti=
on_nr)
>>> +{
>>> +    while (++section_nr <=3D __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 =3D pfn_to_section_nr(++pfn);
>>>=20
>>> -    section_nr =3D pfn_to_section_nr(++pfn);
>>>   if (present_section_nr(section_nr))
>>>       return pfn;
>>> -
>>> -    while (++section_nr <=3D __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));
>>=20
>> 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.
>>=20
>> Have you considered the case?
>=20
> Yes, see the patch description. We return -1 << PFN_SECTION_SHIFT, so a nu=
mber 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=E2=80=98s late in Germany, will double check tomorrow :)=