Re: [PATCH] mm/mempolicy: recompute automatic interleave weights on memory hotplug

[Gregory Price]

On Mon, Oct 26, 2026 at 12:19:21PM +0100, David Hildenbrand (Arm) wrote:
> > +/* Recompute the weights from node_bw_table, unless they were set manually */
> > +static int wi_state_refresh(void)
> > +{
> > +	struct weighted_interleave_state *new_wi_state, *old_wi_state;
> > +	int i;
> > +
> > +	new_wi_state = kmalloc_flex(*new_wi_state, iw_table, nr_node_ids);
> > +	if (!new_wi_state)
> > +		return -ENOMEM;
> > +	new_wi_state->mode_auto = true;
> > +	for (i = 0; i < nr_node_ids; i++)
> > +		new_wi_state->iw_table[i] = 1;
> > +
> > +	mutex_lock(&wi_state_lock);
> > +	old_wi_state = rcu_dereference_protected(wi_state,
> > +					lockdep_is_held(&wi_state_lock));
> > +	/* NULL wi_state assumes auto=true */
> > +	if (!node_bw_table || (old_wi_state && !old_wi_state->mode_auto)) {
> 
> 
> I was confused about the node_bw_table check at first. Maybe
> 
> /*
>  * We can get hotplug events before any BW data was recorded.
>  * ...
>  */
> 

I went back and forth on whether to do the mutex in or outside this
function, but ultimately decided inside is just better structurally.

But yes, this comment seems warranted in this case.

> > +	/*
> > +	 * HMAT and CXL report a node's bandwidth only when it first gains
> > +	 * memory, so recompute the weights whenever N_MEMORY changes.
> > +	 */
> > +	wi_state_refresh();
> 
> If the function fails, should we at least pr_warn_once() ?
>

The only failure state is -ENOMEM, wasn't sure that was worth warning
on.  I actually debated whether to just make this a void function.

I don't have a strong feeling here, but happy to add a warn if you
prefer it.

~Gregory