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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Kirill Tkhai <ktkhai@virtuozzo.com>, akpm@linux-foundation.org,
        peterz@infradead.org, oleg@redhat.com, viro@zeniv.linux.org.uk,
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        keescook@chromium.org, riel@redhat.com, tglx@linutronix.de,
        kirill.shutemov@linux.intel.com, marcos.souza.org@gmail.com,
        hoeun.ryu@gmail.com, pasha.tatashin@oracle.com, gs051095@gmail.com,
        dhowells@redhat.com, rppt@linux.vnet.ibm.com,
        linux-kernel@vger.kernel.org
References: <152473763015.29458.1131542311542381803.stgit@localhost.localdomain>
        <20180426130700.GP17484@dhcp22.suse.cz>
Date:   Thu, 31 May 2018 20:07:28 -0500
In-Reply-To: <20180426130700.GP17484@dhcp22.suse.cz> (Michal Hocko's message
        of "Thu, 26 Apr 2018 15:07:00 +0200")
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Subject: Re: [PATCH 0/4] exit: Make unlikely case in mm_update_next_owner() more scalable
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Michal Hocko <mhocko@kernel.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