[PATCH 04/26] buffer: Add get_nth_bh()

["Matthew Wilcox (Oracle)" <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

Extract this useful helper from nilfs_page_get_nth_block()

Signed-off-by: Matthew Wilcox (Oracle) <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
---
 fs/nilfs2/page.h            |  7 +------
 include/linux/buffer_head.h | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/fs/nilfs2/page.h b/fs/nilfs2/page.h
index 21ddcdd4d63e..344d71942d36 100644
--- a/fs/nilfs2/page.h
+++ b/fs/nilfs2/page.h
@@ -55,12 +55,7 @@ unsigned long nilfs_find_uncommitted_extent(struct inode *inode,
 static inline struct buffer_head *
 nilfs_page_get_nth_block(struct page *page, unsigned int count)
 {
-	struct buffer_head *bh = page_buffers(page);
-
-	while (count-- > 0)
-		bh = bh->b_this_page;
-	get_bh(bh);
-	return bh;
+	return get_nth_bh(page_buffers(page), count);
 }
 
 #endif /* _NILFS_PAGE_H */
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 1001244a8941..9fc615ee17fd 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -457,6 +457,28 @@ __bread(struct block_device *bdev, sector_t block, unsigned size)
 	return __bread_gfp(bdev, block, size, __GFP_MOVABLE);
 }
 
+/**
+ * get_nth_bh - Get a reference on the n'th buffer after this one.
+ * @bh: The buffer to start counting from.
+ * @count: How many buffers to skip.
+ *
+ * This is primarily useful for finding the nth buffer in a folio; in
+ * that case you pass the head buffer and the byte offset in the folio
+ * divided by the block size.  It can be used for other purposes, but
+ * it will wrap at the end of the folio rather than returning NULL or
+ * proceeding to the next folio for you.
+ *
+ * Return: The requested buffer with an elevated refcount.
+ */
+static inline struct buffer_head *get_nth_bh(struct buffer_head *bh,
+		unsigned int count)
+{
+	while (count--)
+		bh = bh->b_this_page;
+	get_bh(bh);
+	return bh;
+}
+
 bool block_dirty_folio(struct address_space *mapping, struct folio *folio);
 
 #ifdef CONFIG_BUFFER_HEAD
-- 
2.40.1