linux-kernel - Re: linux-5.13.2: warning from kernel/rcu/tree

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Date:   Mon, 19 Jul 2021 19:50:07 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Boqun Feng <boqun.feng@...il.com>,
        Zhouyi Zhou <zhouzhouyi@...il.com>, <paulmck@...nel.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, Chris Clayton <chris2553@...glemail.com>,
        Chris Rankin <rankincj@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        rcu <rcu@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        "Huang, Ying" <ying.huang@...el.com>, <gregkh@...uxfoundation.org>
Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359

On 2021/7/19 19:22, Matthew Wilcox wrote:
> On Mon, Jul 19, 2021 at 07:12:58PM +0800, Miaohe Lin wrote:
>> When in the commit 2799e77529c2a, we're using the percpu_ref to serialize against
>> concurrent swapoff, i.e. there's percpu_ref inside get_swap_device() instead of
>> rcu_read_lock(). Please see commit 63d8620ecf93 ("mm/swapfile: use percpu_ref to
>> serialize against concurrent swapoff") for detail.
> 
> Oh, so this is a backport problem.  2799e77529c2 was backported without
> its prerequisite 63d8620ecf93.  Greg, probably best to just drop

Yes, they're posted as a patch set:

https://lkml.kernel.org/r/20210426123316.806267-1-linmiaohe@huawei.com

> 2799e77529c2 from all stable trees; the race described is not very
> important (swapoff vs reading a page back from that swap device).
> .
> 

The swapoff races with reading a page back from that swap device should be really
uncommon as most users only do swapoff when the system is going to shutdown.

Sorry for the trouble!