Patch "writeback: keep superblock pinned during cgroup writeback association switches" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

writeback: keep superblock pinned during cgroup writeback association switches

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
writeback-keep-superblock-pinned-during-cgroup-writeback-association-switches.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Tue, 16 Feb 2016 11:34:07 -0700
Subject: writeback: keep superblock pinned during cgroup writeback association switches

From: Tejun Heo <[email protected]>

commit 5ff8eaac1636bf6deae86491f4818c4c69d1a9ac upstream.

If cgroup writeback is in use, an inode is associated with a cgroup for writeback. If the inode's main dirtier changes to another cgroup, the association gets updated asynchronously. Nothing was pinning the superblock while such switches are in progress and superblock could go away while async switching is pending or in progress leading to crashes like the following.

kernel BUG at fs/jbd2/transaction.c:319!
invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC

 CPU: 1 PID: 29158 Comm: kworker/1:10 Not tainted 4.5.0-rc3 #51
 Hardware name: Google Google, BIOS Google 01/01/2011
 Workqueue: events inode_switch_wbs_work_fn
 task: ffff880213dbbd40 ti: ffff880209264000 task.ti: ffff880209264000
 RIP: 0010:[<ffffffff803e6922>]  [<ffffffff803e6922>] start_this_handle+0x382/0x3e0
 RSP: 0018:ffff880209267c30  EFLAGS: 00010202
 ...
 Call Trace:
  [<ffffffff803e6be4>] jbd2__journal_start+0xf4/0x190
  [<ffffffff803cfc7e>] __ext4_journal_start_sb+0x4e/0x70
  [<ffffffff803b31ec>] ext4_evict_inode+0x12c/0x3d0
  [<ffffffff8035338b>] evict+0xbb/0x190
  [<ffffffff80354190>] iput+0x130/0x190
  [<ffffffff80360223>] inode_switch_wbs_work_fn+0x343/0x4c0
  [<ffffffff80279819>] process_one_work+0x129/0x300
  [<ffffffff80279b16>] worker_thread+0x126/0x480
  [<ffffffff8027ed14>] kthread+0xc4/0xe0
  [<ffffffff809771df>] ret_from_fork+0x3f/0x70

Fix it by bumping s_active while cgroup association switching is in flight.

index 6915c950e6e8..1f76d8950a57 100644
@@ -317,6 +317,7 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 	struct inode_switch_wbs_context *isw =
 		container_of(work, struct inode_switch_wbs_context, work);
 	struct inode *inode = isw->inode;
+	struct super_block *sb = inode->i_sb;
 	struct address_space *mapping = inode->i_mapping;
 	struct bdi_writeback *old_wb = inode->i_wb;
 	struct bdi_writeback *new_wb = isw->new_wb;

@@ -423,6 +424,7 @@ skip_switch:
 	wb_put(new_wb);
 
 	iput(inode);
+	deactivate_super(sb);
 	kfree(isw);
 }
 

@@ -469,11 +471,14 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 
 	/* while holding I_WB_SWITCH, no one else can update the association */
 	spin_lock(&inode->i_lock);
+
 	if (inode->i_state & (I_WB_SWITCH | I_FREEING) ||
-	    inode_to_wb(inode) == isw->new_wb) {
-		spin_unlock(&inode->i_lock);
-		goto out_free;
-	}
+	    inode_to_wb(inode) == isw->new_wb)
+		goto out_unlock;
+
+	if (!atomic_inc_not_zero(&inode->i_sb->s_active))
+		goto out_unlock;
+
 	inode->i_state |= I_WB_SWITCH;
 	spin_unlock(&inode->i_lock);
 

@@ -489,6 +494,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
 	return;
 
+out_unlock:
+	spin_unlock(&inode->i_lock);
 out_free:
 	if (isw->new_wb)
 		wb_put(isw->new_wb);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/writeback-keep-superblock-pinned-during-cgroup-writeback-association-switches.patch queue-4.4/workqueue-handle-numa_no_node-for-unbound-pool_workqueue-lookup.patch queue-4.4/cgroup-make-sure-a-parent-css-isn-t-offlined-before-its-children.patch queue-4.4/cpuset-make-mm-migration-asynchronous.patch queue-4.4/libata-fix-sff-host-state-machine-locking-while-polling.patch queue-4.4/revert-workqueue-make-sure-delayed-work-run-in-local-cpu.patch