linux-kernel - Re: [RFC PATCH 1/3] memcg-v1: fully deprecate move_charge_at

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Message-ID: <Zxp7ItxIf744tFbD@google.com>
Date: Thu, 24 Oct 2024 16:51:46 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Michal Hocko <mhocko@...e.com>
Cc: Shakeel Butt <shakeel.butt@...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 1/3] memcg-v1: fully deprecate
 move_charge_at_immigrate

On Thu, Oct 24, 2024 at 11:14:01AM +0200, Michal Hocko wrote:
> On Wed 23-10-24 23:57:10, Shakeel Butt wrote:
> > Proceed with the complete deprecation of memcg v1's charge moving
> > feature. The deprecation warning has been in the kernel for almost two
> > years and has been ported to all stable kernel since. Now is the time to
> > fully deprecate this feature.
> > 
> > Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>
> 
> I fine with this move, just one detail we might need to consider
> [...]
> > @@ -606,17 +606,7 @@ static int mem_cgroup_move_charge_write(struct cgroup_subsys_state *css,
> >  		     "Please report your usecase to linux-mm@...ck.org if you "
> >  		     "depend on this functionality.\n");
> >  
> > -	if (val & ~MOVE_MASK)
> > -		return -EINVAL;
> > -
> > -	/*
> > -	 * No kind of locking is needed in here, because ->can_attach() will
> > -	 * check this value once in the beginning of the process, and then carry
> > -	 * on with stale data. This means that changes to this value will only
> > -	 * affect task migrations starting after the change.
> > -	 */
> > -	memcg->move_charge_at_immigrate = val;
> > -	return 0;
> > +	return -EINVAL;
> 
> Would it make more sense to -EINVAL only if val != 0? The reason being
> that some userspace might be just writing 0 here for whatever reason and
> see the failure unexpected.

I think it's a good idea.

Thanks!