Re: [PATCH for-4.2 2/3] block, dm: don't copy bios for request clones

[Christoph Hellwig <hch@lst.de>]

On Wed, May 27, 2015 at 09:50:18AM +0000, Junichi Nomura wrote:
> Can you test this scenario with your patch?
>   1. Set up a multipath device with fail-over mode
>   2. Write something to the multipath device.
>      After the clone request is sent to the primary path
>      and before the data goes to the disk, 
>      down the primary path
>      (e.g. echo offline > /sys/block/sdXX/device/state)
>   3. (dm-mpath will retry from the secondary path and
>       the write will eventually succeed)
>   4. Verify if the written data is really on the disk

Verified as not working correctly.  The patch below fixes it,
but it needs more testing and some comments:

diff --git a/block/blk-core.c b/block/blk-core.c
index aa819a5..54feaae 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -117,7 +117,13 @@ EXPORT_SYMBOL(blk_rq_init);
 static void req_bio_endio(struct request *rq, struct bio *bio,
 			  unsigned int nbytes, int error)
 {
-	if (error && !(rq->cmd_flags & REQ_CLONE))
+	if (rq->cmd_flags & REQ_CLONE) {
+		if (!error && test_bit(BIO_UPTODATE, &bio->bi_flags))
+			bio_advance(bio, nbytes);
+		return;
+	}
+
+	if (error)
 		clear_bit(BIO_UPTODATE, &bio->bi_flags);
 	else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
 		error = -EIO;
@@ -128,8 +134,7 @@ static void req_bio_endio(struct request *rq, struct bio *bio,
 	bio_advance(bio, nbytes);
 
 	/* don't actually finish bio if it's part of flush sequence */
-	if (bio->bi_iter.bi_size == 0 &&
-	    !(rq->cmd_flags & (REQ_FLUSH_SEQ|REQ_CLONE)))
+	if (bio->bi_iter.bi_size == 0 && !(rq->cmd_flags & REQ_FLUSH_SEQ))
 		bio_endio(bio, error);
 }