linux-kernel - Re: [PATCH v3 1/6] mm: memcontrol: introduce mem_cgroup_ino() and mem_cgroup_get_from

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Date:   Tue, 24 May 2022 10:00:26 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Dave Chinner <dchinner@...hat.com>,
        linux-kernel@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: Re: [PATCH v3 1/6] mm: memcontrol: introduce mem_cgroup_ino() and
 mem_cgroup_get_from_ino()

On Mon, May 23, 2022 at 11:12:12AM -0700, Roman Gushchin wrote:
> On Sun, May 22, 2022 at 03:05:33PM +0800, Muchun Song wrote:
> > On Mon, May 09, 2022 at 11:38:15AM -0700, Roman Gushchin wrote:
> > > Shrinker debugfs requires a way to represent memory cgroups without
> > > using full paths, both for displaying information and getting input
> > > from a user.
> > > 
> > > Cgroup inode number is a perfect way, already used by bpf.
> > > 
> > > This commit adds a couple of helper functions which will be used
> > > to handle memcg-aware shrinkers.
> > > 
> > > Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > > ---
> > >  include/linux/memcontrol.h | 21 +++++++++++++++++++++
> > >  mm/memcontrol.c            | 23 +++++++++++++++++++++++
> > >  2 files changed, 44 insertions(+)
> > > 
> > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > > index fe580cb96683..a6de9e5c1549 100644
> > > --- a/include/linux/memcontrol.h
> > > +++ b/include/linux/memcontrol.h
> > > @@ -831,6 +831,15 @@ static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
> > >  }
> > >  struct mem_cgroup *mem_cgroup_from_id(unsigned short id);
> > >  
> > > +#ifdef CONFIG_SHRINKER_DEBUG
> > > +static inline unsigned long mem_cgroup_ino(struct mem_cgroup *memcg)
> > > +{
> > > +	return memcg ? cgroup_ino(memcg->css.cgroup) : 0;
> > > +}
> > > +
> > > +struct mem_cgroup *mem_cgroup_get_from_ino(unsigned long ino);
> > > +#endif
> > > +
> > >  static inline struct mem_cgroup *mem_cgroup_from_seq(struct seq_file *m)
> > >  {
> > >  	return mem_cgroup_from_css(seq_css(m));
> > > @@ -1324,6 +1333,18 @@ static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
> > >  	return NULL;
> > >  }
> > >  
> > > +#ifdef CONFIG_SHRINKER_DEBUG
> > > +static inline unsigned long mem_cgroup_ino(struct mem_cgroup *memcg)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > > +static inline struct mem_cgroup *mem_cgroup_get_from_ino(unsigned long ino)
> > > +{
> > > +	return NULL;
> > > +}
> > > +#endif
> > > +
> > >  static inline struct mem_cgroup *mem_cgroup_from_seq(struct seq_file *m)
> > >  {
> > >  	return NULL;
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 04cea4fa362a..e6472728fa66 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -5018,6 +5018,29 @@ struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
> > >  	return idr_find(&mem_cgroup_idr, id);
> > >  }
> > >  
> > > +#ifdef CONFIG_SHRINKER_DEBUG
> > > +struct mem_cgroup *mem_cgroup_get_from_ino(unsigned long ino)
> > > +{
> > > +	struct cgroup *cgrp;
> > > +	struct cgroup_subsys_state *css;
> > > +	struct mem_cgroup *memcg;
> > > +
> > > +	cgrp = cgroup_get_from_id(ino);
> > > +	if (!cgrp)
> > > +		return ERR_PTR(-ENOENT);
> > > +
> > > +	css = cgroup_get_e_css(cgrp, &memory_cgrp_subsys);
> > > +	if (css)
> > > +		memcg = container_of(css, struct mem_cgroup, css);
> > > +	else
> > > +		memcg = ERR_PTR(-ENOENT);
> > > +
> > > +	cgroup_put(cgrp);
> > 
> > I think it's better to use css_put() here since the refcount is get
> > via cgroup_get_e_css() which returns a css struct.
> 
> cgroup_put() is matching cgroup_get_from_id().
> 
> The reference grabbed by cgroup_get_e_css() shouldn't be dropped
> because mem_cgroup_get_from_ino() has a "get" semantics.
>

My bad. I have misread it here. Thanks.