blkcg: associate blkg when associating a device

Previously, blkg association was handled by controller specific code in
blk-throttle and blk-iolatency. However, because a blkg represents a
relationship between a blkcg and a request_queue, it makes sense to keep
the blkg->q and bio->bi_disk->queue consistent.

This patch moves association into the bio_set_dev macro(). This should
cover the majority of cases where the device is set/changed keeping the
two pointers consistent. Fallback code is added to
blkcg_bio_issue_check() to catch any missing paths.
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -2074,6 +2074,7 @@ void bio_associate_blkg(struct bio *bio)
 
 	rcu_read_unlock();
 }
+EXPORT_SYMBOL_GPL(bio_associate_blkg);
 
 /**
  * bio_disassociate_task - undo bio_associate_current()

block/blk-iolatency.c

@@ -472,14 +472,12 @@ static void check_scale_change(struct iolatency_grp *iolat)
 static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio)
 {
 	struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
-	struct blkcg_gq *blkg;
+	struct blkcg_gq *blkg = bio->bi_blkg;
 	bool issue_as_root = bio_issue_as_root_blkg(bio);
 
 	if (!blk_iolatency_enabled(blkiolat))
 		return;
 
-	bio_associate_blkg(bio);
-	blkg = bio->bi_blkg;
 	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
 
 	while (blkg && blkg->parent) {

block/blk-throttle.c

@@ -2118,7 +2118,6 @@ static inline void throtl_update_latency_buckets(struct throtl_data *td)
 static void blk_throtl_assoc_bio(struct bio *bio)
 {
 #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
-	bio_associate_blkg(bio);
 	bio_issue_init(&bio->bi_issue, bio_sectors(bio));
 #endif
 }

include/linux/bio.h

@@ -491,12 +491,14 @@ do {						\
 		bio_clear_flag(bio, BIO_THROTTLED);\
 	(bio)->bi_disk = (bdev)->bd_disk;	\
 	(bio)->bi_partno = (bdev)->bd_partno;	\
+	bio_associate_blkg(bio);		\
 } while (0)
 
 #define bio_copy_dev(dst, src)			\
 do {						\
 	(dst)->bi_disk = (src)->bi_disk;	\
 	(dst)->bi_partno = (src)->bi_partno;	\
+	bio_clone_blkcg_association(dst, src);	\
 } while (0)
 
 #define bio_dev(bio) \

include/linux/blk-cgroup.h

@@ -21,6 +21,7 @@
 #include <linux/blkdev.h>
 #include <linux/atomic.h>
 #include <linux/kthread.h>
+#include <linux/fs.h>
 
 /* percpu_counter batch for blkg_[rw]stats, per-cpu drift doesn't matter */
 #define BLKG_STAT_CPU_BATCH	(INT_MAX / 2)
@@ -802,21 +803,23 @@ static inline bool blk_throtl_bio(struct request_queue *q, struct blkcg_gq *blkg
 static inline bool blkcg_bio_issue_check(struct request_queue *q,
 					 struct bio *bio)
 {
-	struct blkcg *blkcg;
 	struct blkcg_gq *blkg;
 	bool throtl = false;
 
-	rcu_read_lock();
+	if (!bio->bi_blkg) {
+		char b[BDEVNAME_SIZE];
+
+		WARN_ONCE(1,
+			  "no blkg associated for bio on block-device: %s\n",
+			  bio_devname(bio, b));
+		bio_associate_blkg(bio);
+	}
 
-	/* associate blkcg if bio hasn't attached one */
-	bio_associate_blkcg(bio, NULL);
-	blkcg = bio_blkcg(bio);
-	blkg = blkg_lookup_create(blkcg, q);
+	blkg = bio->bi_blkg;
 
 	throtl = blk_throtl_bio(q, blkg, bio);
 
 	if (!throtl) {
-		blkg = blkg ?: q->root_blkg;
 		/*
 		 * If the bio is flagged with BIO_QUEUE_ENTERED it means this
 		 * is a split bio and we would have already accounted for the
@@ -828,7 +831,6 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 		blkg_rwstat_add(&blkg->stat_ios, bio->bi_opf, 1);
 	}
 
-	rcu_read_unlock();
 	return !throtl;
 }