linux-kernel - Re: [PATCH v2] sched/core: Don't use dying mm as active

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Message-ID: <20190729091249.GE9330@dhcp22.suse.cz>
Date:   Mon, 29 Jul 2019 11:12:49 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Phil Auld <pauld@...hat.com>
Subject: Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of
 kthreads

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?

> 
> 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.
-- 
Michal Hocko
SUSE Labs