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Date:   Tue, 16 Jul 2019 14:10:48 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Ralph Campbell <rcampbell@nvidia.com>
Cc:     John Hubbard <jhubbard@nvidia.com>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>,
        =?ISO-8859-1?Q?J=E9r=F4me?= Glisse <jglisse@redhat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Jason Gunthorpe <jgg@mellanox.com>
Subject: Re: [PATCH] mm/hmm: Fix bad subpage pointer in try_to_unmap_one
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On Mon, 15 Jul 2019 17:38:04 -0700 Ralph Campbell <rcampbell@nvidia.com> wrote:

> I'm not surprised at the confusion. It took me quite awhile to 
> understand how migrate_vma() works with ZONE_DEVICE private memory.
>
> ...
> 
> I see Christoph Hellwig got confused by this too [1].

While making such discoveries, please prepare a patch which adds
comments which will help the next poor soul!