From: "Dennis Zhou (Facebook)" <dennisszhou@xxxxxxxxx>
There are several scenarios where blkg_lookup_create can fail. Examples
include the blkcg dying, request_queue is dying, or simply being OOM. At
the end of the day, most handle this by simply falling back to the
q->root_blkg and calling it a day.
This patch implements the notion of closest blkg. During
blkg_lookup_create, if it fails to create, return the closest blkg
found or the q->root_blkg. blkg_try_get_closest is introduced and used
during association so a bio is always attached to a blkg.
Signed-off-by: Dennis Zhou <dennisszhou@xxxxxxxxx>
---
block/bio.c | 4 +---
block/blk-cgroup.c | 22 +++++++++++++++-------
include/linux/blk-cgroup.h | 14 ++++++++++++++
3 files changed, 30 insertions(+), 10 deletions(-)
diff --git a/block/bio.c b/block/bio.c
index 91c6b1458454..cd947ad4cf08 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -2021,9 +2021,7 @@ int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)
{
if (unlikely(bio->bi_blkg))
return -EBUSY;
- if (!blkg_try_get(blkg))
- return -ENODEV;
- bio->bi_blkg = blkg;
+ bio->bi_blkg = blkg_try_get_closest(blkg);
return 0;
}
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index cd0d97bed83d..9f8aba29b7c1 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -268,9 +268,8 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg,
* 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.
*
- * 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).
+ * Returns the blkg or the closest blkg if blkg_create fails as it walks
+ * down from root.
*/
struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
struct request_queue *q)
@@ -285,7 +284,7 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
* we shouldn't allow anything to go through for a bypassing queue.
*/
if (unlikely(blk_queue_bypass(q)))
- return ERR_PTR(blk_queue_dying(q) ? -ENODEV : -EBUSY);
+ return q->root_blkg;
blkg = __blkg_lookup(blkcg, q, true);
if (blkg)
@@ -298,14 +297,23 @@ struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
while (true) {
struct blkcg *pos = blkcg;
struct blkcg *parent = blkcg_parent(blkcg);
-
- while (parent && !__blkg_lookup(parent, q, false)) {
+ struct blkcg_gq *ret_blkg = NULL;
+
+ while (parent) {
+ blkg = __blkg_lookup(parent, q, false);
+ if (blkg) {
+ /* remember closest blkg */
+ ret_blkg = blkg;
+ break;
+ }
pos = parent;
parent = blkcg_parent(parent);
}
blkg = blkg_create(pos, q, NULL);
- if (pos == blkcg || IS_ERR(blkg))
+ if (IS_ERR(blkg))
+ return ret_blkg ?: q->root_blkg;
+ if (pos == blkcg)
return blkg;
}
}
diff --git a/include/linux/blk-cgroup.h b/include/linux/blk-cgroup.h
index cc0f238530f6..1fbff1bbb651 100644
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -549,6 +549,20 @@ static inline struct blkcg_gq *blkg_try_get(struct blkcg_gq *blkg)
return NULL;
}
+/**
+ * blkg_try_get_closest - try and get a blkg ref on the closet blkg
+ * @blkg: blkg to get
+ *
+ * This walks up the blkg tree to find the closest non-dying blkg and returns
+ * the blkg that it did association with as it may not be the passed in blkg.
+ */
+static inline struct blkcg_gq *blkg_try_get_closest(struct blkcg_gq *blkg)
+{
+ while (!atomic_inc_not_zero(&blkg->refcnt))
+ blkg = blkg->parent;
+
+ return blkg;
+}
void __blkg_release_rcu(struct rcu_head *rcu);
--
2.17.1
