Re: [PATCH v3 3/4] mm/page_owner: Print memcg information

[Waiman Long]

On 2/1/22 05:54, Michal Hocko wrote:
> On Mon 31-01-22 14:23:07, Waiman Long wrote:
> > It was found that a number of offlined 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
> > offlined 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.
> >
> > In order to find out more information about those pages that pin
> > offlined memcgs, the page_owner feature is extended to print memory
> > cgroup information especially whether the cgroup is offlined or not.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > ---
> >   mm/page_owner.c | 39 +++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 39 insertions(+)
> >
> > diff --git a/mm/page_owner.c b/mm/page_owner.c
> > index 28dac73e0542..a471c74c7fe0 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 +326,42 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
> >   	seq_putc(m, '\n');
> >   }
> >   
> > +#ifdef CONFIG_MEMCG
> > +/*
> > + * Looking for memcg information and print it out
> > + */
> > +static inline void print_page_owner_memcg(char *kbuf, size_t count, int *pret,
> > +					  struct page *page)
> > +{
> > +	unsigned long memcg_data = READ_ONCE(page->memcg_data);
> > +	struct mem_cgroup *memcg;
> > +	bool onlined;
> > +	char name[80];
> > +
> > +	if (!memcg_data)
> > +		return;
> > +
> > +	if (memcg_data & MEMCG_DATA_OBJCGS)
> > +		*pret += scnprintf(kbuf + *pret, count - *pret,
> > +				"Slab cache page\n");
> > +
> > +	memcg = page_memcg_check(page);
> > +	if (!memcg)
> > +		return;
> > +
> > +	onlined = (memcg->css.flags & CSS_ONLINE);
> > +	cgroup_name(memcg->css.cgroup, name, sizeof(name));
> > +	*pret += scnprintf(kbuf + *pret, count - *pret,
> > +			"Charged %sto %smemcg %s\n",
> > +			PageMemcgKmem(page) ? "(via objcg) " : "",
> > +			onlined ? "" : "offlined ",
> > +			name);
> I have asked in the previous version already but what makes the memcg
> stable (why it cannot go away and be reallocated for something else)
> while you are trying to get its name?

The memcg is not going away as long as the page isn't freed unless if it 
is indirectly connected via objcg. Of course, there can be a race 
between the page is going to be freed while the page_owner information 
is being displayed. One solution is to add a simple bit lock to each of 
the page_owner structure and acquire the lock when it is being written 
to or read from. Anyway a lot of these debugging aids or tools don't 
eliminate all the race conditions that affect the accuracy of the 
displayed information. I can add a patch to eliminate this direct memcg 
race if you think this is necessary.

Cheers,
Longman