blkcg: remove additional reference to the css

The previous patch in this series removed carrying around a pointer to
the css in blkg. However, the blkg association logic still relied on
taking a reference on the css to ensure we wouldn't fail in getting a
reference for the blkg.

Here the implicit dependency on the css is removed. The association
continues to rely on the tryget logic walking up the blkg tree. This
streamlines the three ways that association can happen: normal, swap,
and writeback.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 28 insertions, 38 deletions

diff --git a/block/bio.c b/block/bio.c
index 2b6bc7b805ec7..ce1e512dca5ac 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1966,8 +1966,6 @@ EXPORT_SYMBOL(bioset_init_from_src);
 void bio_disassociate_blkg(struct bio *bio)
 {
 	if (bio->bi_blkg) {
-		/* a ref is always taken on css */
-		css_put(&bio_blkcg(bio)->css);
 		blkg_put(bio->bi_blkg);
 		bio->bi_blkg = NULL;
 	}
@@ -1995,33 +1993,31 @@ static void __bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)
 	bio->bi_blkg = blkg_try_get_closest(blkg);
 }
 
-static void __bio_associate_blkg_from_css(struct bio *bio,
-					  struct cgroup_subsys_state *css)
-{
-	struct blkcg_gq *blkg;
-
-	rcu_read_lock();
-
-	blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_disk->queue);
-	__bio_associate_blkg(bio, blkg);
-
-	rcu_read_unlock();
-}
-
 /**
  * bio_associate_blkg_from_css - associate a bio with a specified css
  * @bio: target bio
  * @css: target css
  *
  * Associate @bio with the blkg found by combining the css's blkg and the
- * request_queue of the @bio.  This takes a reference on the css that will
- * be put upon freeing of @bio.
+ * request_queue of the @bio.  This falls back to the queue's root_blkg if
+ * the association fails with the css.
  */
 void bio_associate_blkg_from_css(struct bio *bio,
 				 struct cgroup_subsys_state *css)
 {
-	css_get(css);
-	__bio_associate_blkg_from_css(bio, css);
+	struct request_queue *q = bio->bi_disk->queue;
+	struct blkcg_gq *blkg;
+
+	rcu_read_lock();
+
+	if (!css || !css->parent)
+		blkg = q->root_blkg;
+	else
+		blkg = blkg_lookup_create(css_to_blkcg(css), q);
+
+	__bio_associate_blkg(bio, blkg);
+
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
 
@@ -2032,8 +2028,8 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
  * @page: the page to lookup the blkcg from
  *
  * Associate @bio with the blkg from @page's owning memcg and the respective
- * request_queue.  This works like every other associate function wrt
- * references.
+ * request_queue.  If cgroup_e_css returns %NULL, fall back to the queue's
+ * root_blkg.
  */
 void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
 {
@@ -2042,8 +2038,12 @@ void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
 	if (!page->mem_cgroup)
 		return;
 
-	css = cgroup_get_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
-	__bio_associate_blkg_from_css(bio, css);
+	rcu_read_lock();
+
+	css = cgroup_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
+	bio_associate_blkg_from_css(bio, css);
+
+	rcu_read_unlock();
 }
 #endif /* CONFIG_MEMCG */
 
@@ -2058,24 +2058,16 @@ void bio_associate_blkg_from_page(struct bio *bio, struct page *page)
  */
 void bio_associate_blkg(struct bio *bio)
 {
-	struct request_queue *q = bio->bi_disk->queue;
-	struct blkcg *blkcg;
-	struct blkcg_gq *blkg;
+	struct cgroup_subsys_state *css;
 
 	rcu_read_lock();
 
 	if (bio->bi_blkg)
-		blkcg = bio->bi_blkg->blkcg;
+		css = &bio_blkcg(bio)->css;
 	else
-		blkcg = css_to_blkcg(blkcg_get_css());
+		css = blkcg_css();
 
-	if (!blkcg->css.parent) {
-		__bio_associate_blkg(bio, q->root_blkg);
-	} else {
-		blkg = blkg_lookup_create(blkcg, q);
-
-		__bio_associate_blkg(bio, blkg);
-	}
+	bio_associate_blkg_from_css(bio, css);
 
 	rcu_read_unlock();
 }
@@ -2097,10 +2089,8 @@ void bio_disassociate_task(struct bio *bio)
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
-	if (src->bi_blkg) {
-		css_get(&bio_blkcg(src)->css);
+	if (src->bi_blkg)
 		__bio_associate_blkg(dst, src->bi_blkg);
-	}
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 #endif /* CONFIG_BLK_CGROUP */