On Wed 30-03-16 18:51:12, Hekuang wrote: > hi > > 在 2016/3/30 18:38, Mel Gorman 写道: > >On Wed, Mar 30, 2016 at 10:22:07AM +0000, He Kuang wrote: > >>This reverts commit 998d39cb236fe464af86a3492a24d2f67ee1efc2. > >> > >>When local irq is disabled, a percpu variable does not change, so we can > >>remove the access macros and let the compiler optimize the code safely. > >> > >batch can be changed from other contexts. Why is this safe? > > > I've mistakenly thought that per_cpu variable can only be accessed by that > cpu. git blame would point you to 998d39cb236f ("mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE"). I haven't looked into the code deeply to confirm this is still the case but it would be a good lead that this is not that simple. ACCESS_ONCE resp. {READ,WRITE}_ONCE are usually quite subtle so I would encourage you or anybody else who try to remove them to study the code and the history deeper before removing them. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>