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Subject: Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359
To:     Matthew Wilcox <willy@infradead.org>
CC:     Boqun Feng <boqun.feng@gmail.com>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>, <paulmck@kernel.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
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        Andrew Morton <akpm@linux-foundation.org>,
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <8058e175-cec5-c243-6499-c1cd4e3c8605@huawei.com>
Date:   Mon, 19 Jul 2021 19:50:07 +0800
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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!