On 04/11/2014 02:31 PM, David Herrmann wrote:
Hi
On Fri, Apr 11, 2014 at 3:43 PM, Tony Battersby tonyb@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