block: implement REQ_FLUSH/FUA based interface for FLUSH/FUA requests

Now that the backend conversion is complete, export sequenced
FLUSH/FUA capability through REQ_FLUSH/FUA flags.  REQ_FLUSH means the
device cache should be flushed before executing the request.  REQ_FUA
means that the data in the request should be on non-volatile media on
completion.

Block layer will choose the correct way of implementing the semantics
and execute it.  The request may be passed to the device directly if
the device can handle it; otherwise, it will be sequenced using one or
more proxy requests.  Devices will never see REQ_FLUSH and/or FUA
which it doesn't support.

Also, unlike the original REQ_HARDBARRIER, REQ_FLUSH/FUA requests are
never failed with -EOPNOTSUPP.  If the underlying device doesn't
support FLUSH/FUA, the block layer simply make those noop.  IOW, it no
longer distinguishes between writeback cache which doesn't support
cache flush and writethrough/no cache.  Devices which have WB cache
w/o flush are very difficult to come by these days and there's nothing
much we can do anyway, so it doesn't make sense to require everyone to
implement -EOPNOTSUPP handling.  This will simplify filesystems and
block drivers as they can drop -EOPNOTSUPP retry logic for barriers.

* QUEUE_ORDERED_* are removed and QUEUE_FSEQ_* are moved into
  blk-flush.c.

* REQ_FLUSH w/o data can also be directly passed to drivers without
  sequencing but some drivers assume that zero length requests don't
  have rq->bio which isn't true for these requests requiring the use
  of proxy requests.

* REQ_COMMON_MASK now includes REQ_FLUSH | REQ_FUA so that they are
  copied from bio to request.

* WRITE_BARRIER is marked deprecated and WRITE_FLUSH, WRITE_FUA and
  WRITE_FLUSH_FUA are added.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 13 insertions, 6 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 76041b614758..352c48627381 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -135,12 +135,13 @@ struct inodes_stat_t {
  *                      immediately after submission. The write equivalent
  *                      of READ_SYNC.
  * WRITE_ODIRECT_PLUG   Special case write for O_DIRECT only.
- * WRITE_BARRIER        Like WRITE_SYNC, but tells the block layer that all
- *                      previously submitted writes must be safely on storage
- *                      before this one is started. Also guarantees that when
- *                      this write is complete, it itself is also safely on
- *                      storage. Prevents reordering of writes on both sides
- *                      of this IO.
+ * WRITE_BARRIER        DEPRECATED. Always fails. Use FLUSH/FUA instead.
+ * WRITE_FLUSH          Like WRITE_SYNC but with preceding cache flush.
+ * WRITE_FUA            Like WRITE_SYNC but data is guaranteed to be on
+ *                      non-volatile media on completion.
+ * WRITE_FLUSH_FUA      Combination of WRITE_FLUSH and FUA. The IO is preceded
+ *                      by a cache flush and data is guaranteed to be on
+ *                      non-volatile media on completion.
  *
  */
 #define RW_MASK                 REQ_WRITE
@@ -158,6 +159,12 @@ struct inodes_stat_t {
 #define WRITE_META              (WRITE | REQ_META)
 #define WRITE_BARRIER           (WRITE | REQ_SYNC | REQ_NOIDLE | REQ_UNPLUG | \
                                  REQ_HARDBARRIER)
+#define WRITE_FLUSH             (WRITE | REQ_SYNC | REQ_NOIDLE | REQ_UNPLUG | \
+                                 REQ_FLUSH)
+#define WRITE_FUA               (WRITE | REQ_SYNC | REQ_NOIDLE | REQ_UNPLUG | \
+                                 REQ_FUA)
+#define WRITE_FLUSH_FUA         (WRITE | REQ_SYNC | REQ_NOIDLE | REQ_UNPLUG | \
+                                 REQ_FLUSH | REQ_FUA)
 
 /*
  * These aren't really reads or writes, they pass down information about