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Date:   Tue, 3 Aug 2021 13:01:09 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Yang Shi <shy828301@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Miaohe Lin <linmiaohe@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Minchan Kim <minchan@kernel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()
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On Tue, Aug 03, 2021 at 04:14:38PM +0800, Huang, Ying wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> > But I REALLY REALLY REALLY want a reproducer.  Right now, I have a hard
> > time believing this, or any of the other races can really happen.
> 
> I think the race is only theoretical too.  Firstly, swapoff is a rare
> operations in practice; secondly, the race window is really small.

So do something to provoke it.  Widen the window.  Put an msleep(1000)
between *pagep = NULL and the call to get_swap_device().  That's assuming
that the swapon/swapoff loop that I proposed doesn't work.  Did you
try it?