linux-kernel - Re: [PATCH 0/4] exit: Make unlikely case in mm_update_next

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Message-ID: <877enj9uwf.fsf@xmission.com>
Date:   Thu, 31 May 2018 20:07:28 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Kirill Tkhai <ktkhai@...tuozzo.com>, akpm@...ux-foundation.org,
        peterz@...radead.org, oleg@...hat.com, viro@...iv.linux.org.uk,
        mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
        keescook@...omium.org, riel@...hat.com, tglx@...utronix.de,
        kirill.shutemov@...ux.intel.com, marcos.souza.org@...il.com,
        hoeun.ryu@...il.com, pasha.tatashin@...cle.com, gs051095@...il.com,
        dhowells@...hat.com, rppt@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] exit: Make unlikely case in mm_update_next_owner() more scalable

Michal Hocko <mhocko@...nel.org> writes:

> On Thu 26-04-18 14:00:19, Kirill Tkhai wrote:
>> This function searches for a new mm owner in children and siblings,
>> and then iterates over all processes in the system in unlikely case.
>> Despite the case is unlikely, its probability growths with the number
>> of processes in the system. The time, spent on iterations, also growths.
>> I regulary observe mm_update_next_owner() in crash dumps (not related
>> to this function) of the nodes with many processes (20K+), so it looks
>> like it's not so unlikely case.
>
> Did you manage to find the pattern that forces mm_update_next_owner to
> slow paths? This really shouldn't trigger very often. If we can fallback
> easily then I suspect that we should be better off reconsidering
> mm->owner and try to come up with something more clever. I've had a
> patch to remove owner few years back. It needed some work to finish but
> maybe that would be a better than try to make non-scalable thing suck
> less.

Reading through the code I just found a trivial pattern that triggers
this.  Create a multi-threaded process.  Have the thread group leader
(the first thread) exit.

This has the potential to be a significant DOS attack if anyone cares.

Eric