Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng task swap

[Michal Hocko <mhocko@suse.com>]

On Thu 03-07-25 13:50:06, Peter Zijlstra wrote:
> On Thu, Jul 03, 2025 at 11:28:46AM +0200, Michal Hocko wrote:
> 
> > But thinking about this some more, this would be racy same as the
> > PF_EXITING check. This is not my area but is this performance sensitive
> > path that couldn't live with the proper find_lock_task_mm?
> 
> find_lock_task_mm() seems eminently unsuitable for accounting --
> iterating the task group is insane.
> 
> Looking at this, the mm_struct lifetimes suck.. task_struct reference
> doesn't help, rcu doesn't help :-(
> 
> Also, whatever the solution it needs to be inside this count_memcg_*()
> nonsense, because nobody wants this overhead, esp. not for something
> daft like accounting.
> 
> My primary desire at this point is to just revert the patch that caused
> this. Accounting just isn't worth it. Esp. not since there is already a
> tracepoint in this path -- people that want to count crap can very well
> get their numbers from that.

I would tend to agree with this. Doing the accounting race free on a
remote task is nasty and if this is a rare event that could be avoided
then it should be just dropped than racy and oops prone.

-- 
Michal Hocko
SUSE Labs