\subsection*{buffer\_head}
The \ds{buffer\_head} data structure holds information about a block buffer in the 
buffer cache.
\SeeModule{include/linux/\\fs.h}
\begin{tscreen}\begin{verbatim}
/* bh state bits */
#define BH_Uptodate  0   /* 1 if the buffer contains valid data      */
#define BH_Dirty     1   /* 1 if the buffer is dirty                 */
#define BH_Lock      2   /* 1 if the buffer is locked                */
#define BH_Req       3   /* 0 if the buffer has been invalidated     */
#define BH_Touched   4   /* 1 if the buffer has been touched (aging) */
#define BH_Has_aged  5   /* 1 if the buffer has been aged (aging)    */
#define BH_Protected 6   /* 1 if the buffer is protected             */
#define BH_FreeOnIO  7   /* 1 to discard the buffer_head after IO    */

struct buffer_head {
  /* First cache line: */
  unsigned long      b_blocknr;    /* block number                   */
  kdev_t             b_dev;        /* device (B_FREE = free)         */
  kdev_t             b_rdev;       /* Real device                    */
  unsigned long      b_rsector;    /* Real buffer location on disk   */
  struct buffer_head *b_next;      /* Hash queue list                */
  struct buffer_head *b_this_page; /* circular list of buffers in one 
                                      page                           */

  /* Second cache line: */
  unsigned long      b_state;      /* buffer state bitmap (above)    */
  struct buffer_head *b_next_free;
  unsigned int       b_count;      /* users using this block         */
  unsigned long      b_size;       /* block size                     */

  /* Non-performance-critical data follows. */
  char               *b_data;      /* pointer to data block          */
  unsigned int       b_list;       /* List that this buffer appears  */
  unsigned long      b_flushtime;  /* Time when this (dirty) buffer
                                    * should be written              */
  unsigned long      b_lru_time;   /* Time when this buffer was 
                                    * last used.                     */
  struct wait_queue  *b_wait;
  struct buffer_head *b_prev;      /* doubly linked hash list        */
  struct buffer_head *b_prev_free; /* doubly linked list of buffers  */
  struct buffer_head *b_reqnext;   /* request queue                  */
};
\end{verbatim}\end{tscreen}
