linux-kernel - Re: [RFC PATCH 3/3] memcg-v1: remove memcg move locking code

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Message-ID: <CAJD7tkYsCev299G=h2r_e6i34+ccdXJYphv-bQbROqOd7Lr1Uw@mail.gmail.com>
Date: Thu, 24 Oct 2024 13:32:53 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Shakeel Butt <shakeel.butt@...ux.dev>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Muchun Song <muchun.song@...ux.dev>, Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org, 
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org, 
	Meta kernel team <kernel-team@...a.com>
Subject: Re: [RFC PATCH 3/3] memcg-v1: remove memcg move locking code

On Thu, Oct 24, 2024 at 12:45 PM Michal Hocko <mhocko@...e.com> wrote:
>
> On Thu 24-10-24 10:26:15, Shakeel Butt wrote:
> > On Thu, Oct 24, 2024 at 04:50:37PM GMT, Roman Gushchin wrote:
> > > On Wed, Oct 23, 2024 at 11:57:12PM -0700, Shakeel Butt wrote:
> > > > The memcg v1's charge move feature has been deprecated. There is no need
> > > > to have any locking or protection against the moving charge. Let's
> > > > proceed to remove all the locking code related to charge moving.
> > > >
> > > > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> > >
> > > Reviewed-by: Roman Gushchin <roman.gushchin@...ux.dev>
> >
> > Thanks Roman for the review. Based on Michal's question, I am planning
> > to keep the RCU locking in the next version of this patch and folowup
> > with clear understanding where we really need RCU and where we don't.
>
> I think it would be safer and easier to review if we drop each RCU
> separately or in smaller batches.

FWIW if we go with this route, I agree with Roman's idea about
replacing folio_memcg_lock()/unlock()
with an explicit rcu_read_lock()/rcu_read_unlock(), and then having
separate patches/series that remove the RCU annotations. If done in a
separate series, we should comment the explicit RCU calls
appropriately to reflect the fact that they should mostly be removed
(or at least re-evaluated).