buffer: add alloc_folio_buffers() helper

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

It removes one call to compound_head() compared to alloc_page_buffers().

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>

fs/buffer.c

@@ -901,6 +901,65 @@ struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
 }
 EXPORT_SYMBOL_GPL(alloc_page_buffers);
 
+/*
+ * 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.
+ *
+ * The retry flag is used to differentiate async IO (paging, swapping)
+ * which may not fail from ordinary buffer allocations.
+ */
+struct buffer_head *alloc_folio_buffers(struct folio *folio, unsigned long size,
+					bool retry)
+{
+	struct buffer_head *bh, *head;
+	gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
+	long offset;
+	struct mem_cgroup *memcg, *old_memcg;
+
+	if (retry)
+		gfp |= __GFP_NOFAIL;
+
+	/* The folio lock pins the memcg */
+	memcg = folio_memcg(folio);
+	old_memcg = set_active_memcg(memcg);
+
+	head = NULL;
+	offset = folio_size(folio);
+	while ((offset -= size) >= 0) {
+		bh = alloc_buffer_head(gfp);
+		if (!bh)
+			goto no_grow;
+
+		bh->b_this_page = head;
+		bh->b_blocknr = -1;
+		head = bh;
+
+		bh->b_size = size;
+
+		/* Link the buffer to its folio */
+		set_bh_folio(bh, folio, offset);
+	}
+out:
+	set_active_memcg(old_memcg);
+	return head;
+/*
+ * In case anything failed, we just free everything we got.
+ */
+no_grow:
+	if (head) {
+		do {
+			bh = head;
+			head = head->b_this_page;
+			free_buffer_head(bh);
+		} while (head);
+	}
+
+	goto out;
+}
+EXPORT_SYMBOL_GPL(alloc_folio_buffers);
+
 static inline void
 link_dev_buffers(struct page *page, struct buffer_head *head)
 {