[PATCH 2/6] shm: add sealing API

[Andy Lutomirski]

On 04/11/2014 02:31 PM, David Herrmann wrote:
> Hi
> 
> On Fri, Apr 11, 2014 at 3:43 PM, Tony Battersby <tonyb at cybernetics.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?

A quick grep of the kernel tree finds exactly zero code paths
incrementing i_mmap_writable outside of mmap and fork.

Or do you mean a different kind of write ref?  What am I missing here?

--Andy