Re: [PATCH 0/4] exit: Make unlikely case in mm_update_next_owner() more scalable

[Eric W. Biederman]

Michal Hocko <mhocko@xxxxxxxxxx> 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