Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE"

[Hekuang]

hi

å 2016/3/31 9:39, Zefan Li åé:
> On 2016/3/31 9:14, Hekuang wrote:
> > Hi
> >
> > å 2016/3/30 19:10, Michal Hocko åé:
> > > On Wed 30-03-16 18:51:12, Hekuang wrote:
> > > > hi
> > > >
> > > > å 2016/3/30 18:38, Mel Gorman åé:
> > > > > 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!