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

[Waiman Long <longman@redhat.com>]

On 2/2/22 03:49, Michal Hocko wrote:
> On Tue 01-02-22 12:04:37, Waiman Long wrote:
>> 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@redhat.com>
>>>> Acked-by: David Rientjes <rientjes@google.com>
>>>> ---
>>>>    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.
> Right. And that means that cgtoup_name can go off the rail and wander
> through memory correct?
>
>> 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.
> I do not really see how a bit lock could prevent memcg from going away.
> On the other hand I think RCU read lock should be sufficient to keep the
> memcg from going away completely.
Using rcu_read_lock() is also what I have been thinking of doing. So I 
will update the patch to add that for safety.
>
>> 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.
> I do not mind inaccurate information. That is natural but reading
> through a freed memory can be really harmfull. So this really need to be
> sorted out.

Thanks for the review.

Cheers,
Longman