linux-kernel - Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj

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Date:   Tue, 20 Oct 2020 13:02:35 +0100
From:   Richard Palethorpe <rpalethorpe@...e.de>
To:     Roman Gushchin <guro@...com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        Johannes Weiner <hannes@...xchg.org>, ltp@...ts.linux.it,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        "Christoph Lameter" <cl@...ux.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from
 charging root

Hello,

Richard Palethorpe <rpalethorpe@...e.de> writes:

> Hello Roman,
>
> Roman Gushchin <guro@...com> writes:
>
>> -		page_counter_init(&memcg->memory, NULL);
>> -		page_counter_init(&memcg->swap, NULL);
>> -		page_counter_init(&memcg->kmem, NULL);
>> -		page_counter_init(&memcg->tcpmem, NULL);
>> +		/*
>> +		 * If use_hierarchy == false, consider all page counters direct
>> +		 * descendants of the corresponding root level counters.
>> +		 */
>> +		page_counter_init(&memcg->memory, &root_mem_cgroup->memory);
>> +		page_counter_init(&memcg->swap, &root_mem_cgroup->swap);
>> +		page_counter_init(&memcg->kmem, &root_mem_cgroup->kmem);
>> +		page_counter_init(&memcg->tcpmem, &root_mem_cgroup->tcpmem);
>> +
>>  		/*
>>  		 * Deeper hierachy with use_hierarchy == false doesn't make
>>  		 * much sense so let cgroup subsystem know about this
>
> Perhaps in this case, where the hierarchy is broken, objcgs should also
> be reparented directly to root? Otherwise it will still be possible to
> underflow the counter in a descendant of root which has use_hierarchy=0,
> but also has children.

Sorry ignore me, parent_mem_cgroup already selects root. So in the case
of a broken hierarchy objcgs are reparented directly to root.

-- 
Thank you,
Richard.