Re: [PATCH v4 09/21] mm: swap: allocate a virtual swap slot for each swapped out page

[Peter Zijlstra]

On Wed, Mar 18, 2026 at 03:29:40PM -0700, Nhat Pham wrote:
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index 62cd7b35a29c9..85cb45022e796 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -86,6 +86,7 @@ enum cpuhp_state {
>  	CPUHP_FS_BUFF_DEAD,
>  	CPUHP_PRINTK_DEAD,
>  	CPUHP_MM_MEMCQ_DEAD,
> +	CPUHP_MM_VSWAP_DEAD,
>  	CPUHP_PERCPU_CNT_DEAD,
>  	CPUHP_RADIX_DEAD,
>  	CPUHP_PAGE_ALLOC,

> +static int vswap_cpu_dead(unsigned int cpu)
> +{
> +	struct vswap_cluster *cluster;
> +	int order;
> +
> +	rcu_read_lock();

nit:
	guard(rcu)();

> +	for (order = 0; order < SWAP_NR_ORDERS; order++) {
> +		cluster = per_cpu(percpu_vswap_cluster.clusters[order], cpu);
> +		if (cluster) {
> +			per_cpu(percpu_vswap_cluster.clusters[order], cpu) = NULL;
> +			spin_lock(&cluster->lock);

This breaks on PREEMPT_RT as this is ran with IRQs disabled. This must
be a raw_spinlock_t.

> +			cluster->cached = false;
> +			if (refcount_dec_and_test(&cluster->refcnt))
> +				vswap_cluster_free(cluster);

And this... below.

> +			spin_unlock(&cluster->lock);
> +		}
> +	}
> +	rcu_read_unlock();
> +
> +	return 0;
> +}

> +static void vswap_cluster_free(struct vswap_cluster *cluster)
> +{
> +	VM_WARN_ON(cluster->count || cluster->cached);
> +	VM_WARN_ON(!spin_is_locked(&cluster->lock));

This is terrible, please use:

	lockdep_assert_held(&cluster->lock);

> +	xa_lock(&vswap_cluster_map);

This is again broken, this cannot be from a DEAD callback with IRQs
disabled.

> +	list_del_init(&cluster->list);
> +	__xa_erase(&vswap_cluster_map, cluster->id);

Strictly speaking this can end up in xas_alloc(), which is again, not
allowed in a DEAD callback.

> +	xa_unlock(&vswap_cluster_map);
> +	rcu_head_init(&cluster->rcu);
> +	kvfree_rcu(cluster, rcu);
> +}