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From:   Daniel Vacek <neelx@redhat.com>
Date:   Thu, 1 Mar 2018 18:24:17 +0100
Message-ID: <CACjP9X9E4d-ew9ZaHzhy2+R6DumYSny2_sRqoQa-n6cOZU3Y1w@mail.gmail.com>
Subject: Re: [PATCH] mm/page_alloc: fix memmap_init_zone pageblock alignment
To:     Michal Hocko <mhocko@kernel.org>
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
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On Thu, Mar 1, 2018 at 4:27 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Thu 01-03-18 16:09:35, Daniel Vacek wrote:
>> 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?

Sorry I forgot to answer this. If they were, the loop won't be
skipping them, right? But it really does not matter here, kernel needs
(some) page structures initialized anyways. And I do not feel
comfortable with removing the VM_BUG_ON(). The initialization is what
changed with commit b92df1de5d28, hence fixing this.

--nX

>> 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