- * Historically, a buffer_head was used to map a single block
- * within a page, and of course as the unit of I/O through the
- * filesystem and block layers. Nowadays the basic I/O unit
- * is the bio, and buffer_heads are used for extracting block
- * mappings (via a get_block_t call), for tracking state within
- * a page (via a page_mapping) and for wrapping bio submission
- * for backward compatibility reasons (e.g. submit_bh).
+ * Keep related fields in common cachelines. The most commonly accessed
+ * field (b_state) goes at the start so the compiler does not generate
+ * indexed addressing for it.