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Date:   Tue, 14 Jan 2020 13:41:19 +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()
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On Tue, Jan 14, 2020 at 12:02:00AM +0100, David Hildenbrand wrote:
> 
> 
> > Am 13.01.2020 um 23:57 schrieb David Hildenbrand <dhildenb@redhat.com>:
> > 
> > ﻿
> > 
> >>> Am 13.01.2020 um 23:41 schrieb Kirill A. Shutemov <kirill@shutemov.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@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?
> > 
> > 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 :)

If the end_pfn happens the be more than -1UL << PFN_SECTION_SHIFT we are
screwed: the pfn is invalid, next_present_section_nr() returns -1, the
next iterartion is on the same pfn and we have endless loop.

The question is whether we can prove end_pfn is always less than
-1UL << PFN_SECTION_SHIFT in any configuration of any arch.

It is not obvious for me.

-- 
 Kirill A. Shutemov