Re: [PATCH v3] mm/page_alloc: detect allocation forbidden by cpuset and bail out early

[Feng Tang <feng.tang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Wed, Sep 15, 2021 at 01:30:27PM +0200, Michal Hocko wrote:
> On Wed 15-09-21 13:32:47, Feng Tang wrote:
> > On Tue, Sep 14, 2021 at 05:30:03PM -0700, David Rientjes wrote:
> [...]
> > > I'm wondering about a single node nodemask, for example, where all 
> > > ZONE_NORMAL memory is hot-removed.
> 
> While this is theoretically possible it is highly unlikely to happen.
> Non movable memory just takes one kernel allocation to prevent any
> hotremove operation to finish. I have to say I was not aware of the
> hotplug callback. It all seems rather suspicious. I will have a look.
> 
> Anyway something worth having covered "just in case". Thanks for
> pointing it out.
>  
> > Thanks for the reminding! Yes, memory hot remove can change the
> > cpuset's effective nodemask, we may need to add similar check inside
> > cpuset_hotplug_update_tasks() which is called by cpuset_hotplug_workfn(), 
> > something like below?
> > 
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index 7fa633e..d5f6776 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -3186,6 +3186,14 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
> >  	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
> >  	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
> >  
> > +	if (mems_updated && !cpusets_insane_config() &&
> > +		movable_only_nodes(new_mems)) {
> > +		static_branch_enable(&cpusets_insane_config_key);
> > +		pr_info("Unsupported (movable nodes only) cpuset configuration detected (nmask=%*pbl) after memory hotplug."
> > +			"Cpuset allocations might fail even with a lot of memory available.\n",
> > +			nodemask_pr_args(new_mems);
> > +	}
> 
> Please create a helper rather than two copies of the same. Thanks!

Sure. Some draft add-on patch below.

Thanks,
Feng

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7fa633e..3bb9f4ea 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -391,6 +391,18 @@ static inline bool is_in_v2_mode(void)
 	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
 }
 
+static inline void check_insane_mems_config(nodemask_t *nodes)
+{
+	if (!cpusets_insane_config() &&
+		movable_only_nodes(nodes)) {
+		static_branch_enable(&cpusets_insane_config_key);
+		pr_info("Unsupported (movable nodes only) cpuset configuration detected (nmask=%*pbl)! "
+			"Cpuset allocations might fail even with a lot of memory available.\n",
+			nodemask_pr_args(nodes));
+	}
+}
+
 /*
  * Return in pmask the portion of a task's cpusets's cpus_allowed that
  * are online and are capable of running the task.  If none are found,
@@ -1875,13 +1887,7 @@ static int update_nodemask(struct cpuset *cs, struct cpuset *trialcs,
 	if (retval < 0)
 		goto done;
 
-	if (!cpusets_insane_config() &&
-		movable_only_nodes(&trialcs->mems_allowed)) {
-		static_branch_enable(&cpusets_insane_config_key);
-		pr_info("Unsupported (movable nodes only) cpuset configuration detected (nmask=%*pbl)! "
-			"Cpuset allocations might fail even with a lot of memory available.\n",
-			nodemask_pr_args(&trialcs->mems_allowed));
-	}
+	check_insane_mems_config(&trialcs->mems_allowed);
 
 	spin_lock_irq(&callback_lock);
 	cs->mems_allowed = trialcs->mems_allowed;
@@ -3186,6 +3192,9 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
 	mems_updated = !nodes_equal(new_mems, cs->effective_mems);
 
+	if (mems_updated)
+		check_insane_mems_config(&new_mems);
+
 	if (is_in_v2_mode())
 		hotplug_update_tasks(cs, &new_cpus, &new_mems,
 				     cpus_updated, mems_updated);