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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 12 Jul 2020 20:51:26 -0700
Message-ID: <CAHk-=whmbpZN6-Q=8cDM42UmHmqzgNDucLLP4BvR1jQ73+KSgw@mail.gmail.com>
Subject: Re: WARNING: at mm/mremap.c:211 move_page_tables in i386
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Naresh Kamboju <naresh.kamboju@linaro.org>,
        linux- stable <stable@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@kernel.org>,
        lkft-triage@lists.linaro.org, Chris Down <chris@chrisdown.name>,
        Michel Lespinasse <walken@google.com>,
        Fan Yang <Fan_Yang@sjtu.edu.cn>,
        Brian Geffon <bgeffon@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>, pugaowei@gmail.com,
        Jerome Glisse <jglisse@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Hugh Dickins <hughd@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>,
        Sasha Levin <sashal@kernel.org>
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On Sun, Jul 12, 2020 at 7:53 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> > But I do feel like you figured out why the bug happened, now we're
> > just discussing whether the patch is the right thing to do.
>
> Yes.
>
> > Maybe saying "doing the pmd copies for the initial stack isn't
> > important, so let's just note this as a special case and get rid of
> > the WARN_ON()" might be an alternative solution.
>
> Personally, I feel it is better to keep the warning just so in the future we
> can detect any bugs.

I don't disagree, the warning didn't happen to find a bug now, but it
did fine a case we might be able to do better.

So now that I feel we understand the issue, and it's not a horrible
problem, just a (very hard to trigger) warning, I don't think there's
any huge hurry.

I think think I will - for now - change the WARN_ON() to
WARN_ON_ONCE() (so that it doesn't floow the logs if somebody triggers
this odd special case  this malisiously), and add a note about how
this happens to the code for posterito.

And if/when you figure out a better way to fix it, we can update the note.

Ok?

             Linus