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, 2 insertions, 1 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 580b513668fe..eb0131319134 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -86,7 +86,8 @@ extern int dir_notify_enable;
 #define READ_META       (READ | (1 << BIO_RW_META))
 #define WRITE_SYNC      (WRITE | (1 << BIO_RW_SYNC))
 #define SWRITE_SYNC     (SWRITE | (1 << BIO_RW_SYNC))
-#define WRITE_BARRIER   ((1 << BIO_RW) | (1 << BIO_RW_BARRIER))
+#define WRITE_BARRIER   (WRITE | (1 << BIO_RW_BARRIER))
+#define WRITE_DISCARD   (WRITE | (1 << BIO_RW_DISCARD))
 
 #define SEL_IN          1
 #define SEL_OUT         2