From: Yu Kuai <yukuai@fygo.io>

blkg_conf_prep() open-codes the same parent walk and blkg creation that
blkg_lookup_create() already performs. Make blkg_lookup_create() report
whether the target blkg was created or found while still returning the
closest existing blkg on failure, then have blkg_conf_prep() use the
helper and treat errors as config failures.

This keeps the bio association path's closest-blkg fallback and removes
the duplicate config path loop.

Signed-off-by: Yu Kuai <yukuai@fygo.io>
---
 block/blk-cgroup.c | 81 +++++++++++++++-------------------------------
 1 file changed, 26 insertions(+), 55 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index ddc9073d7ab9..ae481bcde934 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -448,17 +448,19 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk,
  * blkg_lookup_create - lookup blkg, try to create one if not there
  * @blkcg: blkcg of interest
  * @disk: gendisk of interest
+ * @gfp_mask: allocation mask to use
+ * @blkgp: out parameter for the target blkg, or closest blkg on failure
  *
  * Lookup blkg for the @blkcg - @disk pair.  If it doesn't exist, try to
  * create one.  blkg creation is performed recursively from blkcg_root such
  * that all non-root blkg's have access to the parent blkg.  This function
  * must be called with @disk->queue->blkcg_mutex held.
  *
- * Returns the blkg or the closest blkg if blkg_create() fails as it walks
- * down from root.
+ * On success, *@blkgp points to the target blkg and 0 is returned.  On
+ * failure, *@blkgp points to the closest blkg and the errno is returned.
  */
-static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-		struct gendisk *disk)
+static int blkg_lookup_create(struct blkcg *blkcg, struct gendisk *disk,
+			      gfp_t gfp_mask, struct blkcg_gq **blkgp)
 {
 	struct request_queue *q = disk->queue;
 	struct blkcg_gq *blkg;
@@ -470,7 +472,8 @@ static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 		    blkg != rcu_dereference(blkcg->blkg_hint))
 			rcu_assign_pointer(blkcg->blkg_hint, blkg);
 		rcu_read_unlock();
-		return blkg;
+		*blkgp = blkg;
+		return 0;
 	}
 	rcu_read_unlock();
 
@@ -497,16 +500,16 @@ static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 		}
 		rcu_read_unlock();
 
-		blkg = blkg_create(pos, disk, GFP_NOIO);
+		blkg = blkg_create(pos, disk, gfp_mask);
 		if (IS_ERR(blkg)) {
-			blkg = ret_blkg;
-			break;
+			*blkgp = ret_blkg;
+			return PTR_ERR(blkg);
+		}
+		if (pos == blkcg) {
+			*blkgp = blkg;
+			return 0;
 		}
-		if (pos == blkcg)
-			break;
 	}
-
-	return blkg;
 }
 
 static void blkg_destroy(struct blkcg_gq *blkg)
@@ -839,46 +842,10 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		goto fail_unlock;
 	}
 
-	blkg = blkg_lookup(blkcg, q);
-	if (blkg)
-		goto success;
-
-	/*
-	 * Create blkgs walking down from blkcg_root to @blkcg, so that all
-	 * non-root blkgs have access to their parents.
-	 */
-	while (true) {
-		struct blkcg *pos = blkcg;
-		struct blkcg *parent;
-
-		parent = blkcg_parent(blkcg);
-		rcu_read_lock();
-		while (parent && !blkg_lookup(parent, q)) {
-			pos = parent;
-			parent = blkcg_parent(parent);
-		}
-		rcu_read_unlock();
-
-		if (!blkcg_policy_enabled(q, pol)) {
-			ret = -EOPNOTSUPP;
-			goto fail_unlock;
-		}
-
-		rcu_read_lock();
-		blkg = blkg_lookup(pos, q);
-		rcu_read_unlock();
-		if (!blkg) {
-			blkg = blkg_create(pos, disk, GFP_NOIO);
-			if (IS_ERR(blkg)) {
-				ret = PTR_ERR(blkg);
-				goto fail_unlock;
-			}
-		}
+	ret = blkg_lookup_create(blkcg, disk, GFP_NOIO, &blkg);
+	if (ret)
+		goto fail_unlock;
 
-		if (pos == blkcg)
-			goto success;
-	}
-success:
 	ctx->blkg = blkg;
 	return 0;
 
@@ -2018,6 +1985,8 @@ static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
 	if (blkg)
 		return blkg;
 	if (nowait) {
+		int ret;
+
 		/*
 		 * mutex_trylock() itself does not sleep, but mutexes still
 		 * follow task-context locking rules.  Keep atomic nowait callers
@@ -2026,9 +1995,11 @@ static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
 		if (!preemptible() || !mutex_trylock(&q->blkcg_mutex))
 			return NULL;
 
-		blkg = blkg_lookup_create(blkcg, bio->bi_bdev->bd_disk,
-					  GFP_ATOMIC);
-		if (blkg)
+		ret = blkg_lookup_create(blkcg, bio->bi_bdev->bd_disk,
+					 GFP_ATOMIC, &blkg);
+		if (ret)
+			blkg = NULL;
+		else if (blkg)
 			blkg = blkg_lookup_tryget(blkg);
 		mutex_unlock(&q->blkcg_mutex);
 
@@ -2040,7 +2011,7 @@ static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
 	 * time, hold lock to create new blkg.
 	 */
 	mutex_lock(&q->blkcg_mutex);
-	blkg = blkg_lookup_create(blkcg, bio->bi_bdev->bd_disk);
+	blkg_lookup_create(blkcg, bio->bi_bdev->bd_disk, GFP_NOIO, &blkg);
 	if (blkg)
 		blkg = blkg_lookup_tryget(blkg);
 	mutex_unlock(&q->blkcg_mutex);
-- 
2.51.0