linux-kernel - Re: [PATCH 1/2] memcg: mm_update_next

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Date: Thu, 27 Jun 2024 10:29:42 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jens Axboe <axboe@...nel.dk>,
	Jinliang Zheng <alexjlzheng@...cent.com>,
	Mateusz Guzik <mjguzik@...il.com>,
	Matthew Wilcox <willy@...radead.org>,
	Tycho Andersen <tandersen@...flix.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] memcg: mm_update_next_owner: kill the "retry" logic

Michal, thanks for looking at this,

On 06/27, Michal Hocko wrote:
>
> On Wed 26-06-24 17:29:24, Oleg Nesterov wrote:
> > @@ -446,7 +463,6 @@ void mm_update_next_owner(struct mm_struct *mm)
> >  {
> >  	struct task_struct *c, *g, *p = current;
> >
> > -retry:
> >  	/*
> >  	 * If the exiting or execing task is not the owner, it's
> >  	 * someone else's problem.
> > @@ -468,16 +484,16 @@ void mm_update_next_owner(struct mm_struct *mm)
> >  	 * Search in the children
> >  	 */
> >  	list_for_each_entry(c, &p->children, sibling) {
> > -		if (c->mm == mm)
> > -			goto assign_new_owner;
> > +		if (c->mm == mm && try_to_set_owner(c, mm))
> > +			goto ret;
>
> You need to unlock tasklist_lock, right? Same for other goto ret.

No. From the patch

	+/* drops tasklist_lock if succeeds */
	+static bool try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
	+{
	+       bool ret = false;
	+
	+       task_lock(tsk);
	+       if (likely(tsk->mm == mm)) {
	+               /* tsk can't pass exit_mm/exec_mmap and exit */
	+               read_unlock(&tasklist_lock);
	                ^^^^^^^^^^^^^^^^^^^^^^^^^^^

try_to_set_owner() drops tasklist right after it verifies that
tsk->mm == mm under task_lock().

> It should still die but it can do so in a better shape.

Agreed!

Oleg.