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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 18 Jan 2018 09:28:48 -0800
Message-ID: <CA+55aFzviumvZcV4a4ddtLu5T_ibCD_Xia+7W5khY3pMQ--jUA@mail.gmail.com>
Subject: Re: [mm 4.15-rc8] Random oopses under memory pressure.
To:     "Luck, Tony" <tony.luck@intel.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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On Thu, Jan 18, 2018 at 9:26 AM, Luck, Tony <tony.luck@intel.com> wrote:
>> Both are real page. But why do you expect pages to be 64-byte alinged?
>> Both are aligned to 64-bit as they suppose to be IIUC.
>
> On a 64-bit kernel sizeof struct page == 64 (after much work by people to
> trim out excess stuff).  So I thought we made sure to align the base address
> of blocks of "struct page" so that every one neatly fits into one cache line.

The bug happens on 32-bit, and a 'struct page' is not 64-byte aligned
there at all.

See my other email about the explanation of why "page1 - page2"
doesn't work when they aren't mutually aligned to the actual size of
the 'struct page'.

             Linus