mm: shmem: only remove inode from swaplist when it's swapped page count is 0

Even if we fail to allocate a swap entry, the inode might have previously
allocated entry and we might take inode containing swap entry off
swaplist.  As a result, try_to_unuse() may enter a potential dead loop to
repeatedly look for inode and clean it's swap entry.  Only take inode off
swaplist when it's swapped page count is 0 to fix the issue.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b487a2d ("mm, swap: simplify folio swap allocation")
Signed-off-by: Kemeng Shi <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Reviewed-by: Kairui Song <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Cc: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kemeng Shi

mm/shmem.c

@@ -1651,8 +1651,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 		BUG_ON(folio_mapped(folio));
 		return swap_writepage(&folio->page, wbc);
 	}
-
-	list_del_init(&info->swaplist);
+	if (!info->swapped)
+		list_del_init(&info->swaplist);
 	mutex_unlock(&shmem_swaplist_mutex);
 	if (nr_pages > 1)
 		goto try_split;