Messages in this thread | | | Subject | Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE" | From | Hekuang <> | Date | Thu, 31 Mar 2016 09:47:08 +0800 |
| |
hi
在 2016/3/31 9:39, Zefan Li 写道: > On 2016/3/31 9:14, Hekuang wrote: >> Hi >> >> 在 2016/3/30 19:10, Michal Hocko 写道: >>> 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. >>> >> Thank you for responding, I've read that commit and related articles and not sending >> mail casually, though you may think it's a stupid patch. I'm a beginner and I think >> sending mails to maillist is a effective way to learn kernel, And, sure i'll be more careful and >> well prepared next time :) >> > pcp->batch can be changed in a different cpu. You may read percpu_pagelist_fraction_sysctl_handler() > to see how that can happen. > > OK. got it!
| |