mm/shmem: fix potential dead loop in shmem_unuse()

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.

Link: https://lkml.kernel.org/r/20250516170939.965736-4-shikemeng@huaweicloud.com
Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 495e661..aeeddf6 100644 (file)
--- 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);
@@ -1523,13 +1524,15 @@ 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 (atomic_dec_and_test(&info->stop_eviction))
                        wake_up_var(&info->stop_eviction);
                if (error)
                        break;
+               if (list_empty(&info->swaplist))
+                       goto start_over;
+               next = list_next_entry(info, swaplist);
+               if (!info->swapped)
+                       list_del_init(&info->swaplist);
        }
        mutex_unlock(&shmem_swaplist_mutex);