linux-kernel - Re: [PATCH v3 2/2] vmscan,cgroup: apply mems

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Message-ID: <aAbbtNhnuleBZdPK@gourry-fedora-PF4VCD3F>
Date: Mon, 21 Apr 2025 19:58:44 -0400
From: Gregory Price <gourry@...rry.net>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Waiman Long <llong@...hat.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...a.com, hannes@...xchg.org, mhocko@...nel.org,
	roman.gushchin@...ux.dev, muchun.song@...ux.dev, tj@...nel.org,
	mkoutny@...e.com, akpm@...ux-foundation.org
Subject: Re: [PATCH v3 2/2] vmscan,cgroup: apply mems_effective to reclaim

On Mon, Apr 21, 2025 at 04:15:49PM -0700, Shakeel Butt wrote:
> On Mon, Apr 21, 2025 at 06:59:20PM -0400, Gregory Price wrote:
> > On Mon, Apr 21, 2025 at 10:39:58AM -0700, Shakeel Butt wrote:
> > > On Sat, Apr 19, 2025 at 08:14:29PM -0400, Waiman Long wrote:
> > > > 
> > > > On 4/19/25 2:48 PM, Shakeel Butt wrote:
> > > > > On Sat, Apr 19, 2025 at 01:38:24AM -0400, Gregory Price wrote:
> > > > > > +bool cpuset_node_allowed(struct cgroup *cgroup, int nid)
> > > > > > +{
> > > > > > +	struct cgroup_subsys_state *css;
> > > > > > +	unsigned long flags;
> > > > > > +	struct cpuset *cs;
> > > > > > +	bool allowed;
> > > > > > +
> > > > > > +	css = cgroup_get_e_css(cgroup, &cpuset_cgrp_subsys);
> > > > > > +	if (!css)
> > > > > > +		return true;
> > > > > > +
> > > > > > +	cs = container_of(css, struct cpuset, css);
> > > > > > +	spin_lock_irqsave(&callback_lock, flags);
> > > > > Do we really need callback_lock here? We are not modifying and I am
> > > > > wondering if simple rcu read lock is enough here (similar to
> > > > > update_nodemasks_hier() where parent's effective_mems is accessed within
> > > > > rcu read lock).
> > > > 
> > > > The callback_lock is required to ensure the stability of the effective_mems
> > > > which may be in the process of being changed if not taken.
> > > 
> > > Stability in what sense? effective_mems will not get freed under us
> > > here or is there a chance for corrupted read here? node_isset() and
> > > nodes_empty() seems atomic. What's the worst that can happen without
> > > callback_lock?
> > 
> > Fairly sure nodes_empty is not atomic, it's a bitmap search.
> 
> For bitmaps smaller than 64 bits, it seems atomic and MAX_NUMNODES seems
> smaller than 64 in all the archs.

Unfortunately, it's config-defined on (NODES_SHIFT) and the max is 1024.

Is there an argument here for ignoring v1 and just doing the bit-check
without the lock?  Is there an easy ifdef way for us to just return true
if it's v1?

~Gregory