On Fri, Mar 09, 2018 at 10:55:32PM -0800, Dan Williams wrote:

Add a generic facility for awaiting an atomic_t to reach a value of 1.

Page reference counts typically need to reach 0 to be considered a
free / inactive page. However, ZONE_DEVICE pages allocated via
devm_memremap_pages() are never 'onlined', i.e. the put_page() typically
done at init time to assign pages to the page allocator is skipped.

These pages will have their reference count elevated > 1 by
get_user_pages() when they are under DMA. In order to coordinate DMA to
these pages vs filesytem operations like hole-punch and truncate the
filesystem-dax implementation needs to capture the DMA-idle event i.e.
the 2 to 1 count transition).

For now, this implementation does not have functional behavior change,
follow-on patches will add waiters for these page-idle events.

Argh, no no no.. That whole wait_for_atomic_t thing is a giant
trainwreck already and now you're making it worse still.

Please have a look here:

  https://lkml.kernel.org/r/20171101190644.chwhfpoz3ywxx2m7@hirez.programming.kicks-ass.net