Re: [cleanup][PATCH mmotm]memcg: don't call mem_cgroup_soft_limit_check() against root cgroup (Re: [BUG?] [PATCH] soft limits and root cgroups)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* nishimura@mxp.nes.nec.co.jp <nishimura@mxp.nes.nec.co.jp> [2009-12-08 10:09:54]:

> hi,
> 
> On Mon, 7 Dec 2009 20:41:16 +0200, "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
> > Currently, mem_cgroup_update_tree() on root cgroup calls only on
> > uncharge, not on charge.
> > 
> > Is it a bug or not?
> > 
> > Patch to fix, if it's a bug:
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 8aa6026..6babef1 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1366,13 +1366,15 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm
> >                         goto nomem;
> >                 }
> >         }
> > +
> > +done:
> >         /*
> >          * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
> >          * if they exceeds softlimit.
> >          */
> >         if (mem_cgroup_soft_limit_check(mem))
> >                 mem_cgroup_update_tree(mem, page);
> > -done:
> > +
> >         return 0;
> >  nomem:
> >         css_put(&mem->css);
> > 
> I think it's not a bug, because softlimit doesn't work for root cgroup.
> (IIUC, it's not disabled to write to <root cgroup>/memory.soft_limit_in_bytes, but
> it has no use because root cgroup doesn't use res_counter.)
> 
> So, I think not to call mem_cgroup_soft_limit_check()(and mem_cgroup_update_tree)
> against root cgroup on uncharge path would be a right fix.
> 
> 
> How about this ?
> ===
> From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> 
> current memory cgroup doesn't use res_counter about root cgroup, so soft limits
> on root cgroup has no use.
> This patch disables writing to <root cgroup>/memory.soft_limit_in_bytes and
> changes uncharge path not to call mem_cgroup_soft_limit_check() against root
> cgroup.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> ---
>  Documentation/cgroups/memory.txt |    1 +
>  mm/memcontrol.c                  |    6 +++++-
>  2 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> index b871f25..e1b5328 100644
> --- a/Documentation/cgroups/memory.txt
> +++ b/Documentation/cgroups/memory.txt
> @@ -413,6 +413,7 @@ NOTE1: Soft limits take effect over a long period of time, since they involve
>         reclaiming memory for balancing between memory cgroups
>  NOTE2: It is recommended to set the soft limit always below the hard limit,
>         otherwise the hard limit will take precedence.
> +NOTE3: We cannot set soft limits on the root cgroup any more.
> 
>  8. TODO
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 661b8c6..0751533 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2056,7 +2056,7 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
>  	mz = page_cgroup_zoneinfo(pc);
>  	unlock_page_cgroup(pc);
> 
> -	if (mem_cgroup_soft_limit_check(mem))
> +	if (!mem_cgroup_is_root(mem) && mem_cgroup_soft_limit_check(mem))
>  		mem_cgroup_update_tree(mem, page);

May be the mem_cgroup_is_root() check should go inside
mem_cgroup_soft_limit_check() for future call sites as well.

>  	/* at swapout, this memcg will be accessed to record to swap */
>  	if (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
> @@ -2787,6 +2787,10 @@ static int mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
>  			ret = mem_cgroup_resize_memsw_limit(memcg, val);
>  		break;
>  	case RES_SOFT_LIMIT:
> +		if (mem_cgroup_is_root(memcg)) { /* Can't set limit on root */
> +			ret = -EINVAL;
> +			break;
> +		}
>  		ret = res_counter_memparse_write_strategy(buffer, &val);
>  		if (ret)
>  			break;
> 

looks good to me

Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
 

-- 
	Balbir

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