Factor out the order calculation into a new helper, which can be reused
by shmem in the following patch.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Daniel Gomez <da.gomez@samsung.com>
---
 include/linux/pagemap.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index bcf0865a38ae..d796c8a33647 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -727,6 +727,16 @@ typedef unsigned int __bitwise fgf_t;
 
 #define FGP_WRITEBEGIN		(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
 
+static inline unsigned int filemap_get_order(size_t size)
+{
+	unsigned int shift = ilog2(size);
+
+	if (shift <= PAGE_SHIFT)
+		return 0;
+
+	return shift - PAGE_SHIFT;
+}
+
 /**
  * fgf_set_order - Encode a length in the fgf_t flags.
  * @size: The suggested size of the folio to create.
@@ -740,11 +750,11 @@ typedef unsigned int __bitwise fgf_t;
  */
 static inline fgf_t fgf_set_order(size_t size)
 {
-	unsigned int shift = ilog2(size);
+	unsigned int order = filemap_get_order(size);
 
-	if (shift <= PAGE_SHIFT)
+	if (!order)
 		return 0;
-	return (__force fgf_t)((shift - PAGE_SHIFT) << 26);
+	return (__force fgf_t)(order << 26);
 }
 
 void *filemap_get_entry(struct address_space *mapping, pgoff_t index);
-- 
2.39.3