block: introduce REQ_FLUSH flag

SCSI-ml needs a way to mark a request as flush request in
q->prepare_flush_fn because it needs to identify them later (e.g. in
q->request_fn or prep_rq_fn).

queue_flush sets REQ_HARDBARRIER in rq->cmd_flags however the block
layer also sends normal REQ_TYPE_FS requests with REQ_HARDBARRIER. So
SCSI-ml can't use REQ_HARDBARRIER to identify flush requests.

We could change the block layer to clear REQ_HARDBARRIER bit before
sending non flush requests to the lower layers. However, intorudcing
the new flag looks cleaner (surely easier).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: James Bottomley <James.Bottomley@suse.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Alasdair G Kergon <agk@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

2 files changed, 3 insertions, 1 deletions

diff --git a/block/blk-barrier.c b/block/blk-barrier.c
index 7c6f4a714687..a3482425c507 100644
--- a/block/blk-barrier.c
+++ b/block/blk-barrier.c
@@ -143,7 +143,7 @@ static void queue_flush(struct request_queue *q, unsigned which)
         }
 
         blk_rq_init(q, rq);
-        rq->cmd_flags = REQ_HARDBARRIER;
+        rq->cmd_flags = REQ_HARDBARRIER | REQ_FLUSH;
         rq->rq_disk = q->bar_rq.rq_disk;
         rq->end_io = end_io;
         q->prepare_flush_fn(q, rq);
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 4d379c8250ae..f655b54c9ef3 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -174,6 +174,7 @@ enum rq_flag_bits {
         __REQ_ALLOCED,          /* request came from our alloc pool */
         __REQ_COPY_USER,        /* contains copies of user pages */
         __REQ_INTEGRITY,        /* integrity metadata has been remapped */
+        __REQ_FLUSH,            /* request for cache flush */
         __REQ_IO_STAT,          /* account I/O stat */
         __REQ_MIXED_MERGE,      /* merge of different types, fail separately */
         __REQ_NR_BITS,          /* stops here */
@@ -213,6 +214,7 @@ enum rq_flag_bits {
 #define REQ_ALLOCED             (1 << __REQ_ALLOCED)
 #define REQ_COPY_USER           (1 << __REQ_COPY_USER)
 #define REQ_INTEGRITY           (1 << __REQ_INTEGRITY)
+#define REQ_FLUSH               (1 << __REQ_FLUSH)
 #define REQ_IO_STAT             (1 << __REQ_IO_STAT)
 #define REQ_MIXED_MERGE         (1 << __REQ_MIXED_MERGE)