On 03/16/2017 09:33 AM, Jens Axboe wrote:

On 03/15/2017 03:51 PM, Goldwyn Rodrigues wrote:

quoted

diff --git a/block/blk-core.c b/block/blk-core.c
index 0eeb99e..2e5cba2 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -2014,7 +2019,7 @@ blk_qc_t generic_make_request(struct bio *bio)
 	do {
 		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
 
-		if (likely(blk_queue_enter(q, false) == 0)) {
+		if (likely(blk_queue_enter(q, bio_flagged(bio, BIO_NOWAIT)) == 0)) {
 			struct bio_list hold;
 			struct bio_list lower, same;
 

@@ -2040,7 +2045,10 @@ blk_qc_t generic_make_request(struct bio *bio)
 			bio_list_merge(&bio_list_on_stack, &same);
 			bio_list_merge(&bio_list_on_stack, &hold);
 		} else {
-			bio_io_error(bio);
+			if (unlikely(bio_flagged(bio, BIO_NOWAIT)))
+				bio_wouldblock_error(bio);
+			else
+				bio_io_error(bio);

This doesn't look right. What if the queue is dying, and BIO_NOWAIT just
happened to be set?

Yes, I need to add a condition here to check for blk_queue_dying(). Thanks.

And you're missing wbt_wait() as well as a blocking point. Ditto in
blk-mq.

wbt_wait() does not apply to WRITE_ODIRECT

quoted

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 159187a..942ce8c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -1518,6 +1518,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 	rq = blk_mq_sched_get_request(q, bio, bio->bi_opf, &data);
 	if (unlikely(!rq)) {
 		__wbt_done(q->rq_wb, wb_acct);
+		if (bio && bio_flagged(bio, BIO_NOWAIT))
+			bio_wouldblock_error(bio);
 		return BLK_QC_T_NONE;
 	}
 

This seems a little fragile now, since not both paths free the bio.

Direct I/O should free the bios in bio_dio_complete(). I am not sure why
it would not free bio here originally, but IIRC, this path is for
bio==NULL only. So, with this patch we would get a rq==NULL here and
hence the bio_wouldblock_error() call.

-- 
Goldwyn