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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
