buffer: add folio_alloc_buffers() helper

Folio version of alloc_page_buffers() helper.  This is required to convert
create_page_buffers() to folio_create_buffers() later in the series.

alloc_page_buffers() has been modified to call folio_alloc_buffers() which
adds one call to compound_head() but folio_alloc_buffers() removes one
call to compound_head() compared to the existing alloc_page_buffers()
implementation.

Link: https://lkml.kernel.org/r/20230417123618.22094-3-p.raghav@samsung.comSigned-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -843,7 +843,7 @@ int remove_inode_buffers(struct inode *inode)
 }
 
 /*
- * Create the appropriate buffers when given a page for data area and
+ * Create the appropriate buffers when given a folio for data area and
  * the size of each buffer.. Use the bh->b_this_page linked list to
  * follow the buffers created.  Return NULL if unable to create more
  * buffers.
@@ -851,7 +851,7 @@ int remove_inode_buffers(struct inode *inode)
  * The retry flag is used to differentiate async IO (paging, swapping)
  * which may not fail from ordinary buffer allocations.
  */
-struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
+struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long size,
 					bool retry)
 {
 	struct buffer_head *bh, *head;
@@ -862,12 +862,12 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 	if (retry)
 		gfp |= __GFP_NOFAIL;
 
-	/* The page lock pins the memcg */
-	memcg = page_memcg(page);
+	/* The folio lock pins the memcg */
+	memcg = folio_memcg(folio);
 	old_memcg = set_active_memcg(memcg);
 
 	head = NULL;
-	offset = PAGE_SIZE;
+	offset = folio_size(folio);
 	while ((offset -= size) >= 0) {
 		bh = alloc_buffer_head(gfp);
 		if (!bh)
@@ -879,8 +879,8 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 
 		bh->b_size = size;
 
-		/* Link the buffer to its page */
-		set_bh_page(bh, page, offset);
+		/* Link the buffer to its folio */
+		folio_set_bh(bh, folio, offset);
 	}
 out:
 	set_active_memcg(old_memcg);
@@ -899,6 +899,13 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 
 	goto out;
 }
+EXPORT_SYMBOL_GPL(folio_alloc_buffers);
+
+struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
+				       bool retry)
+{
+	return folio_alloc_buffers(page_folio(page), size, retry);
+}
 EXPORT_SYMBOL_GPL(alloc_page_buffers);
 
 static inline void

include/linux/buffer_head.h

@@ -199,6 +199,8 @@ void set_bh_page(struct buffer_head *bh,
 void folio_set_bh(struct buffer_head *bh, struct folio *folio,
 		  unsigned long offset);
 bool try_to_free_buffers(struct folio *);
+struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long size,
+					bool retry);
 struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 		bool retry);
 void create_empty_buffers(struct page *, unsigned long,