blk-cgroup: Pre-allocate tree node on blkg_conf_prep

Similarly to commit 457e490f2b741 ("blkcg: allocate struct blkcg_gq
outside request queue spinlock"), blkg_create can also trigger
occasional -ENOMEM failures at the radix insertion because any
allocation inside blkg_create has to be non-blocking, making it more
likely to fail.  This causes trouble for userspace tools trying to
configure io weights who need to deal with this condition.

This patch reduces the occurrence of -ENOMEMs on this path by preloading
the radix tree element on a GFP_KERNEL context, such that we guarantee
the later non-blocking insertion won't fail.

A similar solution exists in blkcg_init_queue for the same situation.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index f9389b7..c68bdf5 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -657,6 +657,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                        goto fail;
                }
 
+               if (radix_tree_preload(GFP_KERNEL)) {
+                       blkg_free(new_blkg);
+                       ret = -ENOMEM;
+                       goto fail;
+               }
+
                rcu_read_lock();
                spin_lock_irq(&q->queue_lock);
 
@@ -664,7 +670,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                if (IS_ERR(blkg)) {
                        ret = PTR_ERR(blkg);
                        blkg_free(new_blkg);
-                       goto fail_unlock;
+                       goto fail_preloaded;
                }
 
                if (blkg) {
@@ -673,10 +679,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                        blkg = blkg_create(pos, q, new_blkg);
                        if (IS_ERR(blkg)) {
                                ret = PTR_ERR(blkg);
-                               goto fail_unlock;
+                               goto fail_preloaded;
                        }
                }
 
+               radix_tree_preload_end();
+
                if (pos == blkcg)
                        goto success;
        }
@@ -686,6 +694,8 @@ success:
        ctx->body = input;
        return 0;
 
+fail_preloaded:
+       radix_tree_preload_end();
 fail_unlock:
        spin_unlock_irq(&q->queue_lock);
        rcu_read_unlock();