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

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Date:   Mon, 29 Jul 2019 16:41:00 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
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 7/29/19 2:58 PM, Michal Hocko wrote:
> On Mon 29-07-19 11:27:35, Waiman Long wrote:
>> 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.
> Could you elaborate more? How can a mm counter (do not confuse with
> mm_users) prevent address space to be torn down on exit?

Many of the resources tied to mm_struct are indeed freed when mm_users
becomes 0 including swap space reservation, I think. I was testing a mm
patch and it did have a missing mmput bug that cause mm_users not going
to 0. I fixed the bug, and with sched patch to speed up the release the
mm_struct, every was fine. I didn't realize that fixing the mm bug is
enough to free the swap space.

Still there are some resources not being free when the mm_count is
non-zero. It is certainly less serious than what I have thought. Sorry
for the confusion.

Cheers,
Longman