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Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of
 kthreads
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Phil Auld <pauld@redhat.com>
References: <20190727171047.31610-1-longman@redhat.com>
 <20190729091249.GE9330@dhcp22.suse.cz>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <556445a2-8912-c017-413c-7a4f36c4b89e@redhat.com>
Date:   Mon, 29 Jul 2019 11:27:35 -0400
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On 7/29/19 5:12 AM, Michal Hocko wrote:
> On Sat 27-07-19 13:10:47, Waiman Long wrote:
>> It was found that a dying mm_struct where the owning task has exited
>> can stay on as active_mm of kernel threads as long as no other user
>> tasks run on those CPUs that use it as active_mm. This prolongs the
>> life time of dying mm holding up memory and other resources like swap
>> space that cannot be freed.
> IIRC use_mm doesn't pin the address space. It only pins the mm_struct
> itself. So what exactly is the problem here?

As explained in my response to Peter, I found that resource like swap
space were depleted even after the exit of the offending program in a
mostly idle system. This patch is to make sure that those resources get
freed after program exit ASAP.

>> Fix that by forcing the kernel threads to use init_mm as the active_mm
>> if the previous active_mm is dying.
>>
>> The determination of a dying mm is based on the absence of an owning
>> task. The selection of the owning task only happens with the CONFIG_MEMCG
>> option. Without that, there is no simple way to determine the life span
>> of a given mm. So it falls back to the old behavior.
> Please don't. We really wont to remove mm->owner long term.

OK, if that is the case, I will need to find an alternative way to
determine if an mm is to be freed soon and perhaps set a flag to
indicate that.

Thanks,
Longman