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Date:   Tue, 15 Oct 2019 08:48:27 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Julia Lawall <julia.lawall@lip6.fr>
Cc:     Jens Axboe <axboe@kernel.dk>, kbuild-all@lists.01.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        linux-block@vger.kernel.org
Subject: [PATCH block/for-linus] blkcg: fix botched pd_prealloc error
 handling in blkcg_activate_policy()
Message-ID: <20191015154827.GK18794@devbig004.ftw2.facebook.com>
References: <alpine.DEB.2.21.1910151232480.2818@hadrien>
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In-Reply-To: <alpine.DEB.2.21.1910151232480.2818@hadrien>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

While fixing ->pd_alloc_fn() bug, ab94b0382d81 ("blkcg: Fix
->pd_alloc_fn() being called with the wrong blkcg on policy
activation") broke the pd_prealloc error handling.

* pd's were freed using kfree().  They should be freed with
  ->pd_free_fn().

* pd_prealloc could be kfree()'d and then ->pd_free_fn()'d again.

* When GFP_KERNEL allocation fails, pinned_blkg wasn't put.

There are also a couple existing issues.

* Each pd is initialized as they get allocated.  If alloc fails, the
  policy will get freed with pd's initialized on it.

* After the above partial failure, the partial pds are not freed.

This patch fixes all of the above issues.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Fixes: ab94b0382d81 ("blkcg: Fix ->pd_alloc_fn() being called with the wrong blkcg on policy activation")
---
 block/blk-cgroup.c |   44 +++++++++++++++++++++++++++++++-------------
 1 file changed, 31 insertions(+), 13 deletions(-)

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1373,13 +1373,14 @@ int blkcg_activate_policy(struct request
 retry:
 	spin_lock_irq(&q->queue_lock);
 
-	/* blkg_list is pushed at the head, reverse walk to init parents first */
+	/* blkg_list is pushed at the head, reverse walk to allocate parents first */
 	list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
 		struct blkg_policy_data *pd;
 
 		if (blkg->pd[pol->plid])
 			continue;
 
+		/* If prealloc matches, use it; otherwise try GFP_NOWAIT */
 		if (blkg == pinned_blkg) {
 			pd = pd_prealloc;
 			pd_prealloc = NULL;
@@ -1389,6 +1390,10 @@ retry:
 		}
 
 		if (!pd) {
+			/*
+			 * GFP_NOWAIT failed.  Free the existing one and
+			 * prealloc for @blkg w/ GFP_KERNEL.
+			 */
 			if (pinned_blkg)
 				blkg_put(pinned_blkg);
 			blkg_get(blkg);
@@ -1396,38 +1401,51 @@ retry:
 
 			spin_unlock_irq(&q->queue_lock);
 
-			kfree(pd_prealloc);
+			if (pd_prealloc)
+				pol->pd_free_fn(pd_prealloc);
 			pd_prealloc = pol->pd_alloc_fn(GFP_KERNEL, q,
 						       blkg->blkcg);
-			if (pd_prealloc) {
+			if (pd_prealloc)
 				goto retry;
-			} else {
-				ret = -ENOMEM;
-				goto out_bypass_end;
-			}
+			else
+				goto enomem;
 		}
 
 		blkg->pd[pol->plid] = pd;
 		pd->blkg = blkg;
 		pd->plid = pol->plid;
-		if (pol->pd_init_fn)
-			pol->pd_init_fn(pd);
 	}
 
-	if (pinned_blkg)
-		blkg_put(pinned_blkg);
-	kfree(pd_prealloc);
+	/* all allocated, init in the same order */
+	if (pol->pd_init_fn)
+		list_for_each_entry_reverse(blkg, &q->blkg_list, q_node)
+			pol->pd_init_fn(blkg->pd[pol->plid]);
 
 	__set_bit(pol->plid, q->blkcg_pols);
 	ret = 0;
 
 	spin_unlock_irq(&q->queue_lock);
-out_bypass_end:
+out:
 	if (queue_is_mq(q))
 		blk_mq_unfreeze_queue(q);
+	if (pinned_blkg)
+		blkg_put(pinned_blkg);
 	if (pd_prealloc)
 		pol->pd_free_fn(pd_prealloc);
 	return ret;
+
+enomem:
+	/* alloc failed, nothing's initialized yet, free everything */
+	spin_lock_irq(&q->queue_lock);
+	list_for_each_entry(blkg, &q->blkg_list, q_node) {
+		if (blkg->pd[pol->plid]) {
+			pol->pd_free_fn(blkg->pd[pol->plid]);
+			blkg->pd[pol->plid] = NULL;
+		}
+	}
+	spin_unlock_irq(&q->queue_lock);
+	ret = -ENOMEM;
+	goto out;
 }
 EXPORT_SYMBOL_GPL(blkcg_activate_policy);