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

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Date:   Mon, 31 Jan 2022 16:43:32 -0500
From:   Waiman Long <longman@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.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>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <guro@...com>, Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v3 3/4] mm/page_owner: Print memcg information


On 1/31/22 15:51, Mike Rapoport wrote:
> On Mon, Jan 31, 2022 at 02:23:07PM -0500, 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@...hat.com>
>> Acked-by: David Rientjes <rientjes@...gle.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");
> Don't we need to check for overflow here?

See my previous patch 2 and the reason I used scnprintf() is that it 
never return a length that is >= the given size. So overflow won't 
happen. The final snprintf() in print_page_owner() will detect buffer 
overflow.


>
>> +
>> +	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);
> Ditto
>
>> +}
>> +#else /* CONFIG_MEMCG */
>> +static inline void print_page_owner_memcg(char *kbuf, size_t count, int *pret,
>> +					  struct page *page) { }
> I think #ifdef inside the print_page_owner_memcg() functions will be
> simpler and clearer.
Yes, I see both styles used in kernel code though this style is probably 
more common. I will keep this unless there is a good reason to do otherwise.
>
>> +#endif /* CONFIG_MEMCG */
>> +
>>   static ssize_t
>>   print_page_owner(char __user *buf, size_t count, unsigned long pfn,
>>   		struct page *page, struct page_owner *page_owner,
>> @@ -365,6 +402,8 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
>>   			migrate_reason_names[page_owner->last_migrate_reason]);
>>   	}
>>   
>> +	print_page_owner_memcg(kbuf, count, &ret, page);
>> +
> ret can go over count here.
> Why not make print_page_owner_memcg() an int so that the call will be
> consistent with other calls in print_page_owner():
>
> 	ret += print_page_owner_memcg(kbuf, count, page);
> 	if (ret >= count)
> 		goto err;

See my comments above.

Cheers,
Longman