[PATCH v2] blkcg: allocate struct blkcg_gq outside request queue spinlock

[Tahsin Erdogan <tahsin@google.com>]

blkg_conf_prep() currently calls blkg_lookup_create() while holding
request queue spinlock. This means allocating memory for struct
blkcg_gq has to be made non-blocking. This causes occasional -ENOMEM
failures in call paths like below:

  pcpu_alloc+0x68f/0x710
  __alloc_percpu_gfp+0xd/0x10
  __percpu_counter_init+0x55/0xc0
  cfq_pd_alloc+0x3b2/0x4e0
  blkg_alloc+0x187/0x230
  blkg_create+0x489/0x670
  blkg_lookup_create+0x9a/0x230
  blkg_conf_prep+0x1fb/0x240
  __cfqg_set_weight_device.isra.105+0x5c/0x180
  cfq_set_weight_on_dfl+0x69/0xc0
  cgroup_file_write+0x39/0x1c0
  kernfs_fop_write+0x13f/0x1d0
  __vfs_write+0x23/0x120
  vfs_write+0xc2/0x1f0
  SyS_write+0x44/0xb0
  entry_SYSCALL_64_fastpath+0x18/0xad

In the code path above, percpu allocator cannot call vmalloc() due to
queue spinlock.

A failure in this call path gives grief to tools which are trying to
configure io weights. We see occasional failures happen shortly after
reboots even when system is not under any memory pressure. Machines
with a lot of cpus are more vulnerable to this condition.

Add a flag to blkg_lookup_create() to indicate whether releasing locks
for memory allocation purposes is okay.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
---
v2:
  Moved blkg creation into blkg_lookup_create() to avoid duplicating
  blkg_lookup_create() logic.

 block/blk-cgroup.c         | 51 +++++++++++++++++++++++++++++++++++++++-------
 include/linux/blk-cgroup.h |  4 ++--
 2 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 295e98c2c8cc..afb16e998bf3 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -258,18 +258,22 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
  * blkg_lookup_create - lookup blkg, try to create one if not there
  * @blkcg: blkcg of interest
  * @q: request_queue of interest
+ * @wait_ok: whether blocking for memory allocations is okay
  *
  * Lookup blkg for the @blkcg - @q 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
  * should be called under RCU read lock and @q->queue_lock.
  *
+ * When @wait_ok is true, rcu and queue locks may be dropped for allocating
+ * memory. In this case, the locks will be reacquired on return.
+ *
  * Returns pointer to the looked up or created blkg on success, ERR_PTR()
  * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
  * dead and bypassing, returns ERR_PTR(-EBUSY).
  */
 struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-				    struct request_queue *q)
+				    struct request_queue *q, bool wait_ok)
 {
 	struct blkcg_gq *blkg;
 
@@ -300,7 +304,30 @@ struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
 			parent = blkcg_parent(parent);
 		}
 
-		blkg = blkg_create(pos, q, NULL);
+		if (wait_ok) {
+			struct blkcg_gq *new_blkg;
+
+			spin_unlock_irq(q->queue_lock);
+			rcu_read_unlock();
+
+			new_blkg = blkg_alloc(pos, q, GFP_KERNEL);
+
+			rcu_read_lock();
+			spin_lock_irq(q->queue_lock);
+
+			if (unlikely(!new_blkg))
+				return ERR_PTR(-ENOMEM);
+
+			if (unlikely(blk_queue_bypass(q))) {
+				blkg_free(new_blkg);
+				return ERR_PTR(blk_queue_dying(q) ?
+							-ENODEV : -EBUSY);
+			}
+
+			blkg = blkg_create(pos, q, new_blkg);
+		} else
+			blkg = blkg_create(pos, q, NULL);
+
 		if (pos == blkcg || IS_ERR(blkg))
 			return blkg;
 	}
@@ -789,6 +816,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 {
 	struct gendisk *disk;
 	struct blkcg_gq *blkg;
+	struct request_queue *q;
 	struct module *owner;
 	unsigned int major, minor;
 	int key_len, part, ret;
@@ -812,18 +840,27 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		return -ENODEV;
 	}
 
+	q = disk->queue;
+
 	rcu_read_lock();
-	spin_lock_irq(disk->queue->queue_lock);
+	spin_lock_irq(q->queue_lock);
 
-	if (blkcg_policy_enabled(disk->queue, pol))
-		blkg = blkg_lookup_create(blkcg, disk->queue);
-	else
+	if (blkcg_policy_enabled(q, pol)) {
+		blkg = blkg_lookup_create(blkcg, q, true /* wait_ok */);
+
+		/*
+		 * blkg_lookup_create() may have dropped and reacquired the
+		 * queue lock. Check policy enabled state again.
+		 */
+		if (!IS_ERR(blkg) && unlikely(!blkcg_policy_enabled(q, pol)))
+			blkg = ERR_PTR(-EOPNOTSUPP);
+	} else
 		blkg = ERR_PTR(-EOPNOTSUPP);
 
 	if (IS_ERR(blkg)) {
 		ret = PTR_ERR(blkg);
 		rcu_read_unlock();
-		spin_unlock_irq(disk->queue->queue_lock);
+		spin_unlock_irq(q->queue_lock);
 		owner = disk->fops->owner;
 		put_disk(disk);
 		module_put(owner);
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index 01b62e7bac74..78067dd59c91 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -172,7 +172,7 @@ extern struct cgroup_subsys_state * const blkcg_root_css;
 struct blkcg_gq *blkg_lookup_slowpath(struct blkcg *blkcg,
 				      struct request_queue *q, bool update_hint);
 struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
-				    struct request_queue *q);
+				    struct request_queue *q, bool wait_ok);
 int blkcg_init_queue(struct request_queue *q);
 void blkcg_drain_queue(struct request_queue *q);
 void blkcg_exit_queue(struct request_queue *q);
@@ -694,7 +694,7 @@ static inline bool blkcg_bio_issue_check(struct request_queue *q,
 	blkg = blkg_lookup(blkcg, q);
 	if (unlikely(!blkg)) {
 		spin_lock_irq(q->queue_lock);
-		blkg = blkg_lookup_create(blkcg, q);
+		blkg = blkg_lookup_create(blkcg, q, false /* wait_ok */);
 		if (IS_ERR(blkg))
 			blkg = NULL;
 		spin_unlock_irq(q->queue_lock);
-- 
2.12.0.rc1.440.g5b76565f74-goog