On Sun, Mar 11, 2018 at 4:27 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > 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 That thread seems to be worried about the object disappearing the moment it's reference count reaches a target. That isn't the case with the memmap / struct page objects for ZONE_DEVICE pages. I understand wait_for_atomic_one() is broken in the general case, but as far as I can see it works fine specifically for ZONE_DEVICE page busy tracking, just not generic object lifetime. -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
