[RFC PATCH] raid5: Add R5_ReadNoMerge flag which prevent bio from merging at block layer

[majianpeng <majianpeng@gmail.com>]

Because bios will merge at block-layer,so bios-error may caused by other
bio which be merged into to the same request.
Using this flag,it will find exactly error-sector and not do redundant
operation like re-write and re-read.

Signed-off-by: majianpeng <majianpeng@gmail.com>
---
 block/blk-core.c   |    8 ++++++++
 drivers/md/raid5.c |   16 +++++++++++++---
 drivers/md/raid5.h |    1 +
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 3c923a7..ee04bfc 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1401,6 +1401,9 @@ void init_request_from_bio(struct request *req, struct bio *bio)
 	if (bio->bi_rw & REQ_RAHEAD)
 		req->cmd_flags |= REQ_FAILFAST_MASK;
 
+	if (unlikely(bio->bi_rw & REQ_NOMERGE))
+		req->cmd_flags |= REQ_NOMERGE;
+
 	req->errors = 0;
 	req->__sector = bio->bi_sector;
 	req->ioprio = bio_prio(bio);
@@ -1428,6 +1431,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
 		goto get_rq;
 	}
 
+	if (unlikely(bio->bi_rw & REQ_NOMERGE)) {
+		spin_lock_irq(q->queue_lock);
+		where = ELEVATOR_INSERT_BACK;
+		goto get_rq;
+	}
 	/*
 	 * Check if we can merge with the plugged list before grabbing
 	 * any locks.
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index d267672..04f78d2 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -632,6 +632,9 @@ static void ops_run_io(struct stripe_head *sh, struct stripe_head_state *s)
 			else
 				bi->bi_sector = (sh->sector
 						 + rdev->data_offset);
+			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
+				bi->bi_rw |= REQ_NOMERGE;
+
 			bi->bi_flags = 1 << BIO_UPTODATE;
 			bi->bi_idx = 0;
 			bi->bi_io_vec[0].bv_len = STRIPE_SIZE;
@@ -1731,7 +1734,9 @@ static void raid5_end_read_request(struct bio * bi, int error)
 			atomic_add(STRIPE_SECTORS, &rdev->corrected_errors);
 			clear_bit(R5_ReadError, &sh->dev[i].flags);
 			clear_bit(R5_ReWrite, &sh->dev[i].flags);
-		}
+		} else if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags))
+			clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
+
 		if (atomic_read(&rdev->read_errors))
 			atomic_set(&rdev->read_errors, 0);
 	} else {
@@ -1773,7 +1778,11 @@ static void raid5_end_read_request(struct bio * bi, int error)
 		else
 			retry = 1;
 		if (retry)
-			set_bit(R5_ReadError, &sh->dev[i].flags);
+			if (test_bit(R5_ReadNoMerge, &sh->dev[i].flags)) {
+				set_bit(R5_ReadError, &sh->dev[i].flags);
+				clear_bit(R5_ReadNoMerge, &sh->dev[i].flags);
+			} else
+				set_bit(R5_ReadNoMerge, &sh->dev[i].flags);
 		else {
 			clear_bit(R5_ReadError, &sh->dev[i].flags);
 			clear_bit(R5_ReWrite, &sh->dev[i].flags);
@@ -4481,7 +4490,8 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
 			conf->retry_read_aligned = raid_bio;
 			return handled;
 		}
-
+		if (likely(raid_bio->bi_size >> 9) > STRIPE_SECTORS)
+			set_bit(R5_ReadNoMerge, &sh->dev[dd_idx].flags);
 		handle_stripe(sh);
 		release_stripe(sh);
 		handled++;
diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
index 2164021..6767d07 100644
--- a/drivers/md/raid5.h
+++ b/drivers/md/raid5.h
@@ -273,6 +273,7 @@ enum r5dev_flags {
 	R5_Wantwrite,
 	R5_Overlap,	/* There is a pending overlapping request
 			 * on this block */
+	R5_ReadNoMerge, /* prevent bio from merging in block-layer */
 	R5_ReadError,	/* seen a read error here recently */
 	R5_ReWrite,	/* have tried to over-write the readerror */
 
-- 
1.7.5.4