Hi Josef,

On Fri, Aug 31, 2018 at 11:30:08AM -0400, Josef Bacik wrote:

On Thu, Aug 30, 2018 at 09:53:44PM -0400, Dennis Zhou wrote:

quoted

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a3eede00d302..c626e1f7cdcd 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c

@@ -2129,8 +2129,9 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
 static void blk_throtl_assoc_bio(struct throtl_grp *tg, struct bio *bio)
 {
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
-	if (bio->bi_css)
-		bio_associate_blkg(bio, tg_to_blkg(tg));
+	/* fallback to root_blkg if we fail to get a blkg ref */
+	if (bio->bi_css && bio_associate_blkg(bio, tg_to_blkg(tg)))
+		bio_associate_blkg(bio, bio->bi_disk->queue->root_blkg);

Except if we've already assocated a blkg this is just extra, can we do

if (bio->bi_css && (bio_associate_blkg(bio, tg_to_blkg(tg)) == -ENODEV))

to make it clear that we're only attaching it to the root if we failed to attach
a blkg at all?  Thanks,

Done in v2.

Thanks,
Dennis