gfs2: don't use buffer_heads in gfs2_allocate_page_backing

Rewrite gfs2_allocate_page_backing to call gfs2_iomap_get_alloc and operate on
struct iomap directly.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/file.c

@@ -363,31 +363,30 @@ static void gfs2_size_hint(struct file *filep, loff_t offset, size_t size)
 }
 
 /**
- * gfs2_allocate_page_backing - Use bmap to allocate blocks
+ * gfs2_allocate_page_backing - Allocate blocks for a write fault
  * @page: The (locked) page to allocate backing for
  *
- * We try to allocate all the blocks required for the page in
- * one go. This might fail for various reasons, so we keep
- * trying until all the blocks to back this page are allocated.
- * If some of the blocks are already allocated, thats ok too.
+ * We try to allocate all the blocks required for the page in one go.  This
+ * might fail for various reasons, so we keep trying until all the blocks to
+ * back this page are allocated.  If some of the blocks are already allocated,
+ * that is ok too.
  */
-
 static int gfs2_allocate_page_backing(struct page *page)
 {
-	struct inode *inode = page->mapping->host;
-	struct buffer_head bh;
-	unsigned long size = PAGE_SIZE;
-	u64 lblock = page->index << (PAGE_SHIFT - inode->i_blkbits);
+	u64 pos = page_offset(page);
+	u64 size = PAGE_SIZE;
 
 	do {
-		bh.b_state = 0;
-		bh.b_size = size;
-		gfs2_block_map(inode, lblock, &bh, 1);
-		if (!buffer_mapped(&bh))
+		struct iomap iomap = { };
+
+		if (gfs2_iomap_get_alloc(page->mapping->host, pos, 1, &iomap))
 			return -EIO;
-		size -= bh.b_size;
-		lblock += (bh.b_size >> inode->i_blkbits);
-	} while(size > 0);
+
+		iomap.length = min(iomap.length, size);
+		size -= iomap.length;
+		pos += iomap.length;
+	} while (size > 0);
+
 	return 0;
 }