linux-kernel - Re: [PATCH v2 2/3] mm/memcg: Simplify mem_cgroup_get

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Date:   Mon, 14 Sep 2020 17:29:04 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Roman Gushchin <guro@...com>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH v2 2/3] mm/memcg: Simplify mem_cgroup_get_max()

On Mon, Sep 14, 2020 at 09:51:26AM -0400, Waiman Long wrote:
> On 9/14/20 7:48 AM, Michal Hocko wrote:
> > On Sun 13-09-20 22:44:51, Waiman Long wrote:
> > > The mem_cgroup_get_max() function used to get memory+swap max from
> > > both the v1 memsw and v2 memory+swap page counters & return the maximum
> > > of these 2 values. This is redundant and it is more efficient to just
> > > get either the v1 or the v2 values depending on which one is currently
> > > in use.
> > > 
> > > Signed-off-by: Waiman Long <longman@...hat.com>
> > > ---
> > >   mm/memcontrol.c | 20 +++++++++-----------
> > >   1 file changed, 9 insertions(+), 11 deletions(-)
> > > 
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 8c74f1200261..ca36bed588d1 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -1633,17 +1633,15 @@ void mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> > >    */
> > >   unsigned long mem_cgroup_get_max(struct mem_cgroup *memcg)
> > >   {
> > > -	unsigned long max;
> > > -
> > > -	max = READ_ONCE(memcg->memory.max);
> > > -	if (mem_cgroup_swappiness(memcg)) {
> > > -		unsigned long memsw_max;
> > > -		unsigned long swap_max;
> > > -
> > > -		memsw_max = memcg->memsw.max;
> > > -		swap_max = READ_ONCE(memcg->swap.max);
> > > -		swap_max = min(swap_max, (unsigned long)total_swap_pages);
> > > -		max = min(max + swap_max, memsw_max);
> > > +	unsigned long max = READ_ONCE(memcg->memory.max);
> > > +
> > > +	if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> > > +		if (mem_cgroup_swappiness(memcg))
> > > +			max += min(READ_ONCE(memcg->swap.max),
> > > +				   (unsigned long)total_swap_pages);
> > > +	} else { /* v1 */
> > > +		if (mem_cgroup_swappiness(memcg))
> > > +			max = memcg->memsw.max;
> > I agree that making v1 vs. v2 distinction here makes the code more
> > obvious. But I do not think your code is correct for v1. In a default
> > state it would lead to max = PAGE_COUNTER_MAX which is not something
> > we want, right?
> > 
> > instead you want
> > 		max += min(READ_ONCE(memcg->memsw.max), total_swap_pages);
> > 
> You are right, it is a bit tricky for v1.
> 
> I will change it to
> 
>     max += min(READ_ONCE(memcg->memsw.max) - max, total_swap_pages):

memsw.max can be smaller than max.

max = min3(max, READ_ONCE(memcg->memsw.max), total_swap_pages)?