Re: [PATCH v3 2/3] mm/memcg: Simplify mem_cgroup_get_max()

[Waiman Long]

On 9/14/20 10:47 AM, Michal Hocko wrote:
> On Mon 14-09-20 10:19:55, 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@xxxxxxxxxx>
> > ---
> >   mm/memcontrol.c | 24 +++++++++++++-----------
> >   1 file changed, 13 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 8c74f1200261..2331d4bc7c4d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1633,17 +1633,19 @@ 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)) {
> > +			unsigned long memsw = READ_ONCE(memcg->memsw.max);
> > +
> > +			if (memsw > max)
> > +				max += min(memsw - max, (unsigned long)total_swap_pages);
> Yes this looks better. But, memsw can never be smaller than the hard
> limit (mem_cgroup_resize_max). I would find it slightly easier to
> understand if you did
> 	/* calculate swap excess capacity from memsw limit*/
> 	unsigned long memsw = READ_ONCE(memcg->memsw.max) - max;
> 	max += min (memsw, total_swap_pages);

Right, I thought it was possible to set memsw lower than mem. It was not 
allowed. So the extra check is unnecessary. Will fix that.

Cheers,
Longman