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

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Message-ID: <ekug3nktxwyppavk6tfrp6uxfk3djhqb36xfkb5cltjriqpq5l@qtuszfrnfvu6>
Date: Mon, 21 Apr 2025 16:15:49 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Gregory Price <gourry@...rry.net>
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 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.

Anyways I am hoping that we can avoid taking a global lock in reclaim
path which will become a source of contention for memory pressure
situations.