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

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Message-ID: <hmvt2ydrluiwmautt63gf4w6kypytvb7dztc2c74b5rt5p4er3@v5an7gu462ff>
Date: Thu, 24 Oct 2024 10:16:58 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Roman Gushchin <roman.gushchin@...ux.dev>
Cc: Michal Hocko <mhocko@...e.com>, 
	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 04:51:46PM GMT, Roman Gushchin wrote:
> 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!

Thanks Michal and Roman for the review and I will make this change in
the next version.