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Date:   Mon, 10 Apr 2023 20:53:17 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Baokun Li <libaokun1@huawei.com>
Cc:     <linux-fsdevel@vger.kernel.org>, <viro@zeniv.linux.org.uk>,
        <brauner@kernel.org>, <jack@suse.cz>, <tj@kernel.org>,
        <dennis@kernel.org>, <adilger.kernel@dilger.ca>,
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        <yangerkun@huawei.com>, <houtao1@huawei.com>,
        <stable@vger.kernel.org>
Subject: Re: [PATCH v2] writeback, cgroup: fix null-ptr-deref write in
 bdi_split_work_to_wbs
Message-Id: <20230410205317.dcb186166b9603eeb876dfda@linux-foundation.org>
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References: <20230410130826.1492525-1-libaokun1@huawei.com>
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On Mon, 10 Apr 2023 21:08:26 +0800 Baokun Li <libaokun1@huawei.com> wrote:

> 
> ...
>
> To solve this problem, percpu_ref_exit() is called under RCU protection
> to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs().
> Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(),
> and skip the current wb if wb_tryget() fails because the wb has already
> been shutdown.
> 
> Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones")
> Fixes: f3b6a6df38aa ("writeback, cgroup: keep list of inodes attached to bdi_writeback")

Two Fixes: is awkward.  The Fixes: serves a guide to which kernel
versions should be patched, but those two commits are six years apart.

So... how far back should this fix be backported?

>  fs/fs-writeback.c | 17 ++++++++++-------
>  mm/backing-dev.c  | 12 ++++++++++--

Jens, which tree do you think should carry this?