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Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of
 kthreads
To:     Qais Yousef <qais.yousef@arm.com>
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>
 <20190729081800.qbamrvsf4rjna656@e107158-lin.cambridge.arm.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <be28b3d2-3f94-806b-874d-db2248a2c3a9@redhat.com>
Date:   Mon, 29 Jul 2019 17:06:35 -0400
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On 7/29/19 4:18 AM, Qais Yousef wrote:
> On 07/27/19 13:10, 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.
>>
>> 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.
> I don't really know a lot about this code, but does the owner field has to
> depend on CONFIG_MEMCG? ie: can't the owner be always set?
>
Yes, the owner field is only used and defined when CONFIG_MEMCG is on.

Cheers,
Longman