Re: [PATCH v2 1/2] mm/swap: Add VM_WARN_ON to isolate_lock_cluster()

[YoungJun Park]

On Mon, Mar 09, 2026 at 04:05:41PM +0800, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@xxxxxxxxxx>
> 
> swap_cluster_alloc_table() assumes that the caller holds the following
> locks:
> ci->lock
> percpu_swap_cluster.lock
> si->global_cluster_lock (required for non-SWP_SOLIDSTATE devices)
> 
> There are five call paths leading to swap_cluster_alloc_table():
> swap_alloc_hibernation_slot->cluster_alloc_swap_entry
> ->alloc_swap_scan_list->isolate_lock_cluster->swap_cluster_alloc_table
> 
> swap_alloc_slow->cluster_alloc_swap_entry->alloc_swap_scan_list
> ->isolate_lock_cluster->swap_cluster_alloc_table
> 
> swap_alloc_hibernation_slot->cluster_alloc_swap_entry
> ->swap_reclaim_full_clusters->isolate_lock_cluster
> ->swap_cluster_alloc_table
> 
> swap_alloc_slow->cluster_alloc_swap_entry->swap_reclaim_full_clusters
> ->isolate_lock_cluster->swap_cluster_alloc_table
> 
> swap_reclaim_work->swap_reclaim_full_clusters->isolate_lock_cluster
> ->swap_cluster_alloc_table
> 
> Other paths correctly acquire the necessary locks before calling
> swap_cluster_alloc_table().
> But swap_reclaim_work() doesn't need do that because
> swap_reclaim_full_clusters() just isolate si->full_clusters that
> the tables of it must be allocated.
> Then isolate_lock_cluster() that is called by
> swap_reclaim_full_clusters() will never call swap_cluster_alloc_table().
> 
> This patch add a VM_WARN_ON to warning if a fill cluster will be handle
> by swap_cluster_alloc_table()
> 
> Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> ---
>  mm/swapfile.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 94af29d1de88..3fc2eb30c187 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -596,6 +596,9 @@ static struct swap_cluster_info *isolate_lock_cluster(
>  	spin_unlock(&si->lock);
>  
>  	if (found && !cluster_table_is_alloced(found)) {
> +		/* Table of full cluster must be allocated. */
> +		VM_WARN_ON(ci->flags == CLUSTER_FLAG_FULL);


As Kairui suggested,
Would a VM_WARN to check if `list == si->free_cluster` be good?
Or perhaps `ci->flags != CLUSTER_FLAG_FREE`?

This check only cares about CLUSTER_FLAG_FULL, so it might be better to
cover other flags except the free flag. 

Thanks! 
Youngjun Park