linux-kernel - Re: [PATCH 2/6] shm: add sealing API

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Message-ID: <CANq1E4T=38VLezGH2XUZ9kc=Vtp6Ca++-ATwmEfaXZS6UrTPig@mail.gmail.com>
Date:	Fri, 11 Apr 2014 23:31:27 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Tony Battersby <tonyb@...ernetics.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 2/6] shm: add sealing API

Hi

On Fri, Apr 11, 2014 at 3:43 PM, Tony Battersby <tonyb@...ernetics.com> wrote:
> Exactly.  For O_DIRECT, that would be the call to get_user_pages_fast()
> from dio_refill_pages() in fs/direct-io.c, which is ultimately called
> from blkdev_direct_IO().

If you drop mmap_sem after pinning a page without taking a write-ref,
you break i_mmap_writable / VM_DENYWRITE. In memfd I rely on
i_mmap_writable to work, same thing is done by exec() (and the old,
now disabled, MAP_DENYWRITE).

I don't know whether I should care. I mean, everyone pinning pages and
writing to it without holding the mmap_sem has to take a write-ref for
each page or it breaks i_mmap_writable. So this seems to be a bug in
direct-IO, not in anyone relying on it, right?

Thanks
David
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