linux-kernel - Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin

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Message-ID: <875ywl4m3v.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Wed, 04 Aug 2021 17:04:20 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Miaohe Lin <linmiaohe@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH] mm,shmem: Fix a typo in shmem_swapin_page()

Hugh Dickins <hughd@...gle.com> writes:

> On Tue, 3 Aug 2021, Matthew Wilcox wrote:
>> On Tue, Aug 03, 2021 at 04:14:38PM +0800, Huang, Ying wrote:
>> > Matthew Wilcox <willy@...radead.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?
>
> I've been doing that swapon/swapoff loop for years, while running kernel
> builds on tmpfs going out to swap; for better or worse on baremetal not VM.
>
> You're right that few will ever need that level of reliability; but it
> has caught problems from time to time, and I do insist on fixing them.
>
> I'm not as insistent as you on wanting a reproducer; and we all take pride
> sometimes in fixing ever more inconceivable bugs.  I'm not against that,
> but it's easy to end up with a fix more dangerous than what it claims to
> fix, rather like with random newbie cleanups.

Yes.  I totally agree, bug fixing is hard.

Best Regards,
Huang, Ying