Re: [PATCH v5 3/4] mm/page_owner: Print memcg information

[Waiman Long]

On 2/8/22 07:13, Michal Hocko wrote:
> On Mon 07-02-22 19:05:31, Waiman Long wrote:
> > It was found that a number of dying memcgs were not freed because
> > they were pinned by some charged pages that were present. Even "echo 1 >
> > /proc/sys/vm/drop_caches" wasn't able to free those pages. These dying
> > but not freed memcgs tend to increase in number over time with the side
> > effect that percpu memory consumption as shown in /proc/meminfo also
> > increases over time.
> I still believe that this is very suboptimal way to debug offline memcgs
> but memcg information can be useful in other contexts and it doesn't
> cost us anything except for an additional output so I am fine with this.
I am planning to have a follow-up patch to add a new debugfs file for 
just printing page information associated with dying memcgs only. It 
will be based on the existing page_owner code, though. So I need to get 
this patch in first.
>   
> > In order to find out more information about those pages that pin
> > dying memcgs, the page_owner feature is extended to print memory
> > cgroup information especially whether the cgroup is dying or not.
> > RCU read lock is taken when memcg is being accessed to make sure
> > that it won't be freed.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > Acked-by: Roman Gushchin <guro@xxxxxx>
> > Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> With few comments/questions below.
>
> > ---
> >   mm/page_owner.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 44 insertions(+)
> >
> > diff --git a/mm/page_owner.c b/mm/page_owner.c
> > index 28dac73e0542..d4c311455753 100644
> > --- a/mm/page_owner.c
> > +++ b/mm/page_owner.c
> > @@ -10,6 +10,7 @@
> >   #include <linux/migrate.h>
> >   #include <linux/stackdepot.h>
> >   #include <linux/seq_file.h>
> > +#include <linux/memcontrol.h>
> >   #include <linux/sched/clock.h>
> >   
> >   #include "internal.h"
> > @@ -325,6 +326,47 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
> >   	seq_putc(m, '\n');
> >   }
> >   
> > +/*
> > + * Looking for memcg information and print it out
> > + */
> I am not sure this is particularly useful comment.
Right, I can remove that.
> > +static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,
> > +					 struct page *page)
> > +{
> > +#ifdef CONFIG_MEMCG
> > +	unsigned long memcg_data;
> > +	struct mem_cgroup *memcg;
> > +	bool dying;
> > +
> > +	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;
> > +
> > +	dying = (memcg->css.flags & CSS_DYING);
> Is there any specific reason why you haven't used mem_cgroup_online?
Not really. However, I think checking for CSS_DYING makes more sense now 
that I using the term "dying".
> > +	ret += scnprintf(kbuf + ret, count - ret,
> > +			"Charged %sto %smemcg ",
> > +			PageMemcgKmem(page) ? "(via objcg) " : "",
> > +			dying ? "dying " : "");
> > +
> > +	/* Write cgroup name directly into kbuf */
> > +	cgroup_name(memcg->css.cgroup, kbuf + ret, count - ret);
> > +	ret += strlen(kbuf + ret);
> cgroup_name should return the length of the path added to the buffer.
I realized that after I sent out the patch. I will remove te redundant 
strlen() in a future update.
> > +	ret += scnprintf(kbuf + ret, count - ret, "\n");
> I do not see any overflow prevention here. I believe you really need to
> check ret >= count after each scnprintf/cgroup_name.

As you have realized, the beauty of using scnprintf() is to not needing 
an overflow check after each invocation.

Cheers,
Longman