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

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Message-ID: <aGZuuxxTI6tFOTWK@tiehlicka>
Date: Thu, 3 Jul 2025 13:51:23 +0200
From: Michal Hocko <mhocko@...e.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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 03-07-25 17:37:23, Chen, Yu C wrote:
> Hi Michal,
> 
> On 7/3/2025 3:18 PM, Michal Hocko wrote:
> > On Thu 03-07-25 00:32:47, Chen Yu wrote:
[...]
> > > +
> > > +		if (p->mm)
> > > +			count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
> > > +	}
> > 
> > Why are you testing for p->mm here? Isn't PF_EXITING test sufficient?
> > A robust way to guarantee non-NULL mm against races when a task is
> > exiting is find_lock_task_mm. Probably too heavy weight for this path.
> 
> I suppose we might only need to grab task_lock(p), check if its mm
> pointer is NULL. If yes, we skip the update of memcg event without
> scanning for a non-NULL mm within the process(as find_lock_task_mm()
> does)? If the mm is non-NULL, we update the memcg event with task_lock(p)
> hold and releases it later.

Why not use find_lock_task_mm if task_lock is acceptable for this code
path?
-- 
Michal Hocko
SUSE Labs