Re: [PATCH 3/5] mm/shmem: Fix potential dead loop in shmem_unuse()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/5/15 00:50, Kemeng Shi wrote:
> If multi shmem_unuse() for different swap type is called concurrently,
> a dead loop could occur as following:
> shmem_unuse(typeA)               shmem_unuse(typeB)
>   mutex_lock(&shmem_swaplist_mutex)
>   list_for_each_entry_safe(info, next, ...)
>    ...
>    mutex_unlock(&shmem_swaplist_mutex)
>    /* info->swapped may drop to 0 */
>    shmem_unuse_inode(&info->vfs_inode, type)
> 
>                                    mutex_lock(&shmem_swaplist_mutex)
>                                    list_for_each_entry(info, next, ...)
>                                     if (!info->swapped)
>                                      list_del_init(&info->swaplist)
> 
>                                    ...
>                                    mutex_unlock(&shmem_swaplist_mutex)
> 
>    mutex_lock(&shmem_swaplist_mutex)
>    /* iterate with offlist entry and encounter a dead loop */
>    next = list_next_entry(info, swaplist);
>    ...
> 
> Restart the iteration if the inode is already off shmem_swaplist list
> to fix the issue.
> 
> Fixes: b56a2d8af9147 ("mm: rid swapoff of quadratic complexity")
> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> ---
>   mm/shmem.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 495e661eb8bb..0fed94c2bc09 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1505,6 +1505,7 @@ int shmem_unuse(unsigned int type)
>   		return 0;
>   
>   	mutex_lock(&shmem_swaplist_mutex);
> +start_over:
>   	list_for_each_entry_safe(info, next, &shmem_swaplist, swaplist) {
>   		if (!info->swapped) {
>   			list_del_init(&info->swaplist);
> @@ -1530,6 +1531,8 @@ int shmem_unuse(unsigned int type)

		next = list_next_entry(info, swaplist);
		if (!info->swapped)
			list_del_init(&info->swaplist);
		if (atomic_dec_and_test(&info->stop_eviction))
			wake_up_var(&info->stop_eviction);

We may still hit the list warning when calling list_del_init() for the 
off-list info->swaplist? So I hope we can add a check for the possible 
off-list:

diff --git a/mm/shmem.c b/mm/shmem.c
index 99327c30507c..f5ae5e2d6fb4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1523,9 +1523,11 @@ int shmem_unuse(unsigned int type)
                 cond_resched();

                 mutex_lock(&shmem_swaplist_mutex);
-               next = list_next_entry(info, swaplist);
-               if (!info->swapped)
-                       list_del_init(&info->swaplist);
+               if (!list_empty(&info->swaplist)) {
+                       next = list_next_entry(info, swaplist);
+                       if (!info->swapped)
+                               list_del_init(&info->swaplist);
+               }
                 if (atomic_dec_and_test(&info->stop_eviction))
                         wake_up_var(&info->stop_eviction);
                 if (error)

>   			wake_up_var(&info->stop_eviction);
>   		if (error)
>   			break;
> +		if (list_empty(&info->swaplist))
> +			goto start_over;
>   	}
>   	mutex_unlock(&shmem_swaplist_mutex);
>