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Date: Tue, 8 Jan 2013 20:23:46 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Andrea Arcangeli <aarcange@redhat.com>
cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Hillf Danton <dhillf@gmail.com>, Dave Jones <davej@redhat.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, Linux-MM <linux-mm@kvack.org>,
        Rik van Riel <riel@redhat.com>
Subject: Re: oops in copy_page_rep()
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On Tue, 8 Jan 2013, Andrea Arcangeli wrote:
> 
> Looking at this, one thing that isn't clear is where the page_count is
> checked in migrate_misplaced_transhuge_page. Ok that it's unable to
> migrate anon transhuge COW shared pages so it doesn't need to mess
> with rmap (the mapcount check makes it safe), but it shouldn't be
> allowed to migrate memory that has gup direct-IO in flight (and that
> can only be detected with a page_count vs mapcount check). Real
> migrate does page_freeze_refs to be safe. Mel comments?

Yes, I protested to Mel about that before the holidays, and he
quickly provided a patch, now in akpm's tree; but checking it again
today, I believe it's still not quite right yet - see other mail.

Hugh