mm: shmem: fix incorrect aligned index when checking conflicts

In the shmem_suitable_orders() function, xa_find() is used to check for
conflicts in the pagecache to select suitable huge orders.  However, when
checking each huge order in every loop, the aligned index is calculated
from the previous iteration, which may cause suitable huge orders to be
missed.

We should use the original index each time in the loop to calculate a new
aligned index for checking conflicts to avoid this issue.

Link: https://lkml.kernel.org/r/07433b0f16a152bffb8cee34934a5c040e8e2ad6.1722404078.git.baolin.wang@linux.alibaba.com
Fixes: e7a2ab7b ("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: Barry Song <21cnbao@gmail.com>
Cc: Gavin Shan <gshan@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/shmem.c

@@ -1681,6 +1681,7 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
 					   unsigned long orders)
 {
 	struct vm_area_struct *vma = vmf->vma;
+	pgoff_t aligned_index;
 	unsigned long pages;
 	int order;
 
@@ -1692,9 +1693,9 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
 	order = highest_order(orders);
 	while (orders) {
 		pages = 1UL << order;
-		index = round_down(index, pages);
-		if (!xa_find(&mapping->i_pages, &index,
-			     index + pages - 1, XA_PRESENT))
+		aligned_index = round_down(index, pages);
+		if (!xa_find(&mapping->i_pages, &aligned_index,
+			     aligned_index + pages - 1, XA_PRESENT))
 			break;
 		order = next_order(&orders, order);
 	}