mm: shmem: fix incorrect index alignment for within_size policy

With enabling the shmem per-size within_size policy, using an incorrect
'order' size to round_up() the index can lead to incorrect i_size checks,
resulting in an inappropriate large orders being returned.

Changing to use '1 << order' to round_up() the index to fix this issue. 
Additionally, adding an 'aligned_index' variable to avoid affecting the
index checks.

Link: https://lkml.kernel.org/r/77d8ef76a7d3d646e9225e9af88a76549a68aab1.1734593154.git.baolin.wang@linux.alibaba.com
Fixes: e7a2ab7 ("mm: shmem: add mTHP support for anonymous shmem")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Baolin Wang

mm/shmem.c

@@ -1689,6 +1689,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
 	unsigned long mask = READ_ONCE(huge_shmem_orders_always);
 	unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
 	unsigned long vm_flags = vma ? vma->vm_flags : 0;
+	pgoff_t aligned_index;
 	bool global_huge;
 	loff_t i_size;
 	int order;
@@ -1723,9 +1724,9 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
 	/* Allow mTHP that will be fully within i_size. */
 	order = highest_order(within_size_orders);
 	while (within_size_orders) {
-		index = round_up(index + 1, order);
+		aligned_index = round_up(index + 1, 1 << order);
 		i_size = round_up(i_size_read(inode), PAGE_SIZE);
-		if (i_size >> PAGE_SHIFT >= index) {
+		if (i_size >> PAGE_SHIFT >= aligned_index) {
 			mask |= within_size_orders;
 			break;
 		}