block: add some comments around the bio read-write flags

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/include/linux/bio.h b/include/linux/bio.h
index fb97221d7c30db17da3ac04d8923bfe0e748b79d..ff5b4cf9e2da71abd05b5fac387322b24c75b911 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -123,13 +123,23 @@ struct bio {
 /*
  * bio bi_rw flags
  *
- * bit 0 -- read (not set) or write (set)
+ * bit 0 -- data direction
+ *     If not set, bio is a read from device. If set, it's a write to device.
  * bit 1 -- rw-ahead when set
  * bit 2 -- barrier
+ *     Insert a serialization point in the IO queue, forcing previously
+ *     submitted IO to be completed before this oen is issued.
  * bit 3 -- fail fast, don't want low level driver retries
  * bit 4 -- synchronous I/O hint: the block layer will unplug immediately
+ *     Note that this does NOT indicate that the IO itself is sync, just
+ *     that the block layer will not postpone issue of this IO by plugging.
  * bit 5 -- metadata request
+ *     Used for tracing to differentiate metadata and data IO. May also
+ *     get some preferential treatment in the IO scheduler
  * bit 6 -- discard sectors
+ *     Informs the lower level device that this range of sectors is no longer
+ *     used by the file system and may thus be freed by the device. Used
+ *     for flash based storage.
  */
 #define BIO_RW         0       /* Must match RW in req flags (blkdev.h) */
 #define BIO_RW_AHEAD   1       /* Must match FAILFAST in req flags */