linux-kernel - Re: [PATCH v4 3/4] mm/page_owner: Print memcg information

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Date:   Mon, 7 Feb 2022 18:20:04 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <guro@...com>, Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v4 3/4] mm/page_owner: Print memcg information

On Thu 03-02-22 14:03:58, Waiman Long wrote:
> On 2/3/22 07:46, Michal Hocko wrote:
> > On Wed 02-02-22 15:30:35, Waiman Long wrote:
> > [...]
> > > +#ifdef CONFIG_MEMCG
> > > +	unsigned long memcg_data;
> > > +	struct mem_cgroup *memcg;
> > > +	bool online;
> > > +	char name[80];
> > > +
> > > +	rcu_read_lock();
> > > +	memcg_data = READ_ONCE(page->memcg_data);
> > > +	if (!memcg_data)
> > > +		goto out_unlock;
> > > +
> > > +	if (memcg_data & MEMCG_DATA_OBJCGS)
> > > +		ret += scnprintf(kbuf + ret, count - ret,
> > > +				"Slab cache page\n");
> > > +
> > > +	memcg = page_memcg_check(page);
> > > +	if (!memcg)
> > > +		goto out_unlock;
> > > +
> > > +	online = (memcg->css.flags & CSS_ONLINE);
> > > +	cgroup_name(memcg->css.cgroup, name, sizeof(name));
> > Is there any specific reason to use another buffer allocated on the
> > stack? Also 80B seems too short to cover NAME_MAX.
> > 
> > Nothing else jumped at me.
> 
> I suppose we can print directly into kbuf with cgroup_name(), but using a
> separate buffer is easier to read and understand. 79 characters should be
> enough for most cgroup names. Some auto-generated names with some kind of
> embedded uuids may be longer than that, but the random sequence of hex
> digits that may be missing do not convey much information for identification
> purpose. We can always increase the buffer length later if it turns out to
> be an issue.

Cutting a name short sounds like a source of confusion and there doesn't
seem to be any good reason for that.
-- 
Michal Hocko
SUSE Labs