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Date:   Thu, 1 Mar 2018 16:27:29 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Daniel Vacek <neelx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Paul Burton <paul.burton@imgtec.com>, stable@vger.kernel.org
Subject: Re: [PATCH] mm/page_alloc: fix memmap_init_zone pageblock alignment
Message-ID: <20180301152729.GM15057@dhcp22.suse.cz>
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On Thu 01-03-18 16:09:35, Daniel Vacek wrote:
[...]
> $ grep 7b7ff000 /proc/iomem
> 7b7ff000-7b7fffff : System RAM
[...]
> After commit b92df1de5d28 machine eventually crashes with:
> 
> BUG at mm/page_alloc.c:1913
> 
> >         VM_BUG_ON(page_zone(start_page) != page_zone(end_page));

This is an important information that should be in the changelog.

> >From registers and stack I digged start_page points to
> ffffe31d01ed8000 (note that this is
> page ffffe31d01edffc0 aligned to pageblock) and I can see this in memory dump:
> 
> crash> kmem -p 77fff000 78000000 7b5ff000 7b600000 7b7fe000 7b7ff000
> 7b800000 7ffff000 80000000
>       PAGE        PHYSICAL      MAPPING       INDEX CNT FLAGS
> ffffe31d01e00000  78000000                0        0  0 0
> ffffe31d01ed7fc0  7b5ff000                0        0  0 0
> ffffe31d01ed8000  7b600000                0        0  0 0    <<<< note

Are those ranges covered by the System RAM as well?

> that nodeid and zonenr are encoded in top bits of page flags which are
> not initialized here, hence the crash :-(
> ffffe31d01edff80  7b7fe000                0        0  0 0
> ffffe31d01edffc0  7b7ff000                0        0  1 1fffff00000000
> ffffe31d01ee0000  7b800000                0        0  1 1fffff00000000
> ffffe31d01ffffc0  7ffff000                0        0  1 1fffff00000000

It is still not clear why not to do the alignment in
memblock_next_valid_pfn rahter than its caller.
-- 
Michal Hocko
SUSE Labs