Add 'discard' request handling

Some block devices benefit from a hint that they can forget the contents
of certain sectors. Add basic support for this to the block core, along
with a 'blkdev_issue_discard()' helper function which issues such
requests.

The caller doesn't get to provide an end_io functio, since
blkdev_issue_discard() will automatically split the request up into
multiple bios if appropriate. Neither does the function wait for
completion -- it's expected that callers won't care about when, or even
_if_, the request completes. It's only a hint to the device anyway. By
definition, the file system doesn't _care_ about these sectors any more.

[With feedback from OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> and
Jens Axboe <jens.axboe@oracle.com]

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

1 files changed, 6 insertions, 2 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 17f1fbdb31bf..1fdfc5621c83 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -149,6 +149,8 @@ struct bio {
  * bit 2 -- barrier
  * bit 3 -- fail fast, don't want low level driver retries
  * bit 4 -- synchronous I/O hint: the block layer will unplug immediately
+ * bit 5 -- metadata request
+ * bit 6 -- discard sectors
  */
 #define BIO_RW          0       /* Must match RW in req flags (blkdev.h) */
 #define BIO_RW_AHEAD    1       /* Must match FAILFAST in req flags */
@@ -156,6 +158,7 @@ struct bio {
 #define BIO_RW_FAILFAST 3
 #define BIO_RW_SYNC     4
 #define BIO_RW_META     5
+#define BIO_RW_DISCARD  6
 
 /*
  * upper 16 bits of bi_rw define the io priority of this bio
@@ -186,13 +189,14 @@ struct bio {
 #define bio_rw_ahead(bio)       ((bio)->bi_rw & (1 << BIO_RW_AHEAD))
 #define bio_rw_meta(bio)        ((bio)->bi_rw & (1 << BIO_RW_META))
 #define bio_empty_barrier(bio)  (bio_barrier(bio) && !bio_has_data(bio))
+#define bio_discard(bio)        ((bio)->bi_rw & (1 << BIO_RW_DISCARD))
 
 static inline unsigned int bio_cur_sectors(struct bio *bio)
 {
         if (bio->bi_vcnt)
                 return bio_iovec(bio)->bv_len >> 9;
-
-        return 0;
+        else /* dataless requests such as discard */
+                return bio->bi_size >> 9;
 }
 
 static inline void *bio_data(struct bio *bio)