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Date:   Tue, 14 Jan 2020 18:52:58 +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 11:49:19AM +0100, David Hildenbrand wrote:
> memmap_init_zone() is called for a physical memory region: pfn + size
> (nr_pages)
> 
> The highest possible PFN you can have is "-1(unsigned long) >>
> PFN_SHIFT". So even if you would want to add the very last section, the
> PFN would still be smaller than -1UL << PFN_SECTION_SHIFT.

PFN_SHIFT? I guess you mean PAGE_SHIFT.

Of course PFN can be more than -1UL >> PAGE_SHIFT. Like on 32-bit x86 with
PAE it is ((1ULL << 36) - 1) >> PAGE_SHIFT. That's the whole reason for
PAE.

The highest possible PFN must fit into phys_addr_t when shifted left by
PAGE_SHIFT and must fit into unsigned long. It's can be -1UL if
phys_addr_t is 64-bit.

Any other limitation I miss?

-- 
 Kirill A. Shutemov