linux-kernel - Re: [PATCH] sched/numa: Fix NULL pointer access to mm

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Message-ID: <20250703115006.GT1613200@noisy.programming.kicks-ass.net>
Date: Thu, 3 Jul 2025 13:50:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Michal Hocko <mhocko@...e.com>
Cc: Chen Yu <yu.c.chen@...el.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org,
	Jirka Hladky <jhladky@...hat.com>,
	Srikanth Aithal <Srikanth.Aithal@....com>,
	Suneeth D <Suneeth.D@....com>, Libo Chen <libo.chen@...cle.com>
Subject: Re: [PATCH] sched/numa: Fix NULL pointer access to mm_struct durng
 task swap

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.