Re: Low overhead patches for the memory cgroup controller (v5)

[KAMEZAWA Hiroyuki]

On Mon, 22 Jun 2009 15:43:43 -0700
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 15 Jun 2009 10:09:00 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> >
> > ...
> > 
> > This patch changes the memory cgroup and removes the overhead associated
> > with accounting all pages in the root cgroup. As a side-effect, we can
> > no longer set a memory hard limit in the root cgroup.
> > 
> > A new flag to track whether the page has been accounted or not
> > has been added as well. Flags are now set atomically for page_cgroup,
> > pcg_default_flags is now obsolete and removed.
> > 
> > ...
> >
> > @@ -1114,9 +1121,22 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
> >  		css_put(&mem->css);
> >  		return;
> >  	}
> > +
> >  	pc->mem_cgroup = mem;
> >  	smp_wmb();
> > -	pc->flags = pcg_default_flags[ctype];
> > +	switch (ctype) {
> > +	case MEM_CGROUP_CHARGE_TYPE_CACHE:
> > +	case MEM_CGROUP_CHARGE_TYPE_SHMEM:
> > +		SetPageCgroupCache(pc);
> > +		SetPageCgroupUsed(pc);
> > +		break;
> > +	case MEM_CGROUP_CHARGE_TYPE_MAPPED:
> > +		ClearPageCgroupCache(pc);
> > +		SetPageCgroupUsed(pc);
> > +		break;
> > +	default:
> > +		break;
> > +	}
> 
> Do we still need the smp_wmb()?
> 
> It's hard to say, because we forgot to document it :(
> 
Sorry for lack of documentation.

pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise,
A routine believes USED bit will see bad pc->mem_cgroup.

I'd like to  add a comment later (againt new mmotm.)

Thanks,
-Kame




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