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Subject: Re: [PATCH block/for-4.5-fixes] writeback: keep superblock pinned
 during cgroup writeback association switches
To: Tejun Heo <tj@kernel.org>, Tahsin Erdogan <tahsin@google.com>
References: <CAAeU0aNCq7LGODvVGRU-oU_o-6enii5ey0p1c26D1ZzYwkDc5A@mail.gmail.com>
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Cc: cgroups@vger.kernel.org, "Theodore Ts'o" <tytso@mit.edu>,
        Nauman Rafique <nauman@google.com>, linux-kernel@vger.kernel.org,
        Jan Kara <jack@suse.com>
From: Jens Axboe <axboe@kernel.dk>
Message-ID: <56C36BB9.5000104@kernel.dk>
Date: Tue, 16 Feb 2016 11:34:33 -0700
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On 02/16/2016 11:24 AM, Tejun Heo wrote:
>  From 586afaa034bec88934bad4eb6ab38ba07031ec5a Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Tue, 16 Feb 2016 13:14:35 -0500
>
> 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.

Added for 4.5.

-- 
Jens Axboe