[PATCH 1/2] mm: shmem: fix incorrect index alignment for within_size policy

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

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.

Fixes: e7a2ab7b3bb5 ("mm: shmem: add mTHP support for anonymous shmem")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Hi Andrew,

These two bugfix patches are based on the mm-hotfixes-unstable branch,
and this patch has a slight conflict with my previous patch set:
"Support large folios for tmpfs". However, I think the conflicts are
easy to resolve. If you need me to rebase and resend the
"Support large folios for tmpfs" patch set, please let me know.
Sorry for the troubles :)
---
 mm/shmem.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index f6fb053ac50d..dec659e84562 100644
--- a/mm/shmem.c
+++ b/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;
 		}
-- 
2.39.3