linux-kernel - Re: [PATCH v2] mm: optimize the redundant loop of mm_update_next

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Date: Tue, 25 Jun 2024 15:21:52 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Michal Hocko <mhocko@...e.com>
Cc: Oleg Nesterov <oleg@...hat.com>, alexjlzheng@...il.com,
 "Eric W. Biederman" <ebiederm@...ssion.com>, brauner@...nel.org,
 axboe@...nel.dk, tandersen@...flix.com, willy@...radead.org,
 mjguzik@...il.com, alexjlzheng@...cent.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: optimize the redundant loop of
 mm_update_next_owner()

On Fri, 21 Jun 2024 10:50:10 +0200 Michal Hocko <mhocko@...e.com> wrote:

> On Thu 20-06-24 19:30:19, Oleg Nesterov wrote:
> > Can't review, I forgot everything about mm_update_next_owner().
> > So I am sorry for the noise I am going to add, feel free to ignore.
> > Just in case, I see nothing wrong in this patch.
> > 
> > I think this deserves a comment to explain that this is optimization
> > for the case we race with the pending mmput(). mm_update_next_owner()
> > checks mm_users at the start.
> > 
> > And. Can we drop tasklist and use rcu_read_lock() before for_each_process?
> > Yes, this will probably need more changes even if possible...
> > 
> > 
> > Or even better. Can't we finally kill mm_update_next_owner() and turn the
> > ugly mm->owner into mm->mem_cgroup ?
> 
> Yes, dropping the mm->owner should be a way to go. Replacing that by
> mem_cgroup sounds like an improvemnt. I have a vague recollection that
> this has some traps on the way. E.g. tasks sharing the mm but living in
> different cgroups. Things have changes since the last time I've checked
> and for example memcg charge migration on task move will be deprecated
> soon so chances are that there are less roadblocks on the way.

I think this was alexjlzheng's first kernel contribution and as such we
might not be hearing from him(?) again.  And that's OK, thanks for the
bug report - it helps Linux.

Meanwhile we have a stalled patch in mm-unstable.  If someone could add
this issue to their todo list, that would be great.