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.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dennis Zhou <dennisszhou@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 36 insertions, 26 deletions

diff --git a/block/bio.c b/block/bio.c
index 71cfe3720ea7..c39251e69447 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1978,18 +1978,30 @@ int bio_associate_blkg(struct bio *bio, struct blkcg_gq *blkg)
         return 0;
 }
 
+/**
+ * __bio_associate_blkg_from_css - internal blkg association function
+ *
+ * This in the core association function that all association paths rely on.
+ * A blkg reference is taken which is released upon freeing of the bio.
+ */
 static int __bio_associate_blkg_from_css(struct bio *bio,
                                          struct cgroup_subsys_state *css)
 {
+        struct request_queue *q = bio->bi_disk->queue;
         struct blkcg_gq *blkg;
+        int ret;
 
         rcu_read_lock();
 
-        blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_disk->queue);
+        if (!css || !css->parent)
+                blkg = q->root_blkg;
+        else
+                blkg = blkg_lookup_create(css_to_blkcg(css), q);
 
-        rcu_read_unlock();
+        ret = bio_associate_blkg(bio, blkg);
 
-        return bio_associate_blkg(bio, blkg);
+        rcu_read_unlock();
+        return ret;
 }
 
 /**
@@ -1998,13 +2010,14 @@ static int __bio_associate_blkg_from_css(struct bio *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.
  */
 int bio_associate_blkg_from_css(struct bio *bio,
                                 struct cgroup_subsys_state *css)
 {
-        css_get(css);
+        if (unlikely(bio->bi_blkg))
+                return -EBUSY;
         return __bio_associate_blkg_from_css(bio, css);
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);
@@ -2016,22 +2029,29 @@ 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.
  *
  * Note: this must be called after bio has an associated device.
  */
 int bio_associate_blkg_from_page(struct bio *bio, struct page *page)
 {
         struct cgroup_subsys_state *css;
+        int ret;
 
         if (unlikely(bio->bi_blkg))
                 return -EBUSY;
         if (!page->mem_cgroup)
                 return 0;
-        css = cgroup_get_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
 
-        return __bio_associate_blkg_from_css(bio, css);
+        rcu_read_lock();
+
+        css = cgroup_e_css(page->mem_cgroup->css.cgroup, &io_cgrp_subsys);
+
+        ret = __bio_associate_blkg_from_css(bio, css);
+
+        rcu_read_unlock();
+        return ret;
 }
 #endif /* CONFIG_MEMCG */
 
@@ -2041,12 +2061,12 @@ int bio_associate_blkg_from_page(struct bio *bio, struct page *page)
  * @bio: target bio
  *
  * Associate @bio with the blkg found from the bio's css and the request_queue.
- * If one is not found, bio_lookup_blkg creates the blkg.
+ * If one is not found, bio_lookup_blkg creates the blkg.  This falls back to
+ * the queue's root_blkg if association fails.
  */
 int bio_associate_create_blkg(struct request_queue *q, struct bio *bio)
 {
-        struct blkcg *blkcg;
-        struct blkcg_gq *blkg;
+        struct cgroup_subsys_state *css;
         int ret = 0;
 
         /* someone has already associated this bio with a blkg */
@@ -2055,15 +2075,9 @@ int bio_associate_create_blkg(struct request_queue *q, struct bio *bio)
 
         rcu_read_lock();
 
-        blkcg = css_to_blkcg(blkcg_get_css());
+        css = blkcg_css();
 
-        if (!blkcg->css.parent) {
-                ret = bio_associate_blkg(bio, q->root_blkg);
-        } else {
-                blkg = blkg_lookup_create(blkcg, q);
-
-                ret = bio_associate_blkg(bio, blkg);
-        }
+        ret = __bio_associate_blkg_from_css(bio, css);
 
         rcu_read_unlock();
         return ret;
@@ -2080,8 +2094,6 @@ void bio_disassociate_task(struct bio *bio)
                 bio->bi_ioc = NULL;
         }
         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;
         }
@@ -2094,10 +2106,8 @@ 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 */