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Subject: Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for
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To:     Jann Horn <jannh@google.com>
CC:     Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
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From:   John Hubbard <jhubbard@nvidia.com>
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On 9/21/20 3:27 PM, Jann Horn wrote:
> On Tue, Sep 22, 2020 at 12:18 AM John Hubbard <jhubbard@nvidia.com> wrote:
>> On 9/21/20 2:55 PM, Jann Horn wrote:
>>> On Mon, Sep 21, 2020 at 11:20 PM Peter Xu <peterx@redhat.com> wrote:
>> ...
> Ah... the documentation you linked implies that FOLL_WRITE should more
> or less imply FOLL_PIN? I didn't realize that.
> 

hmmm, that does seem like a pretty close approximation. It's certainly
true that if we were only doing reads, and also never marking pages
dirty, that the file system writeback code would be OK.

For completeness we should add: even just reading a page is still a
problem, if one also marks the page as dirty (which is inconsistent and
wrong, but still). That's because the file system code can then break,
during writeback in particular.


thanks,
-- 
John Hubbard
NVIDIA