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[209.85.167.48]) by smtp.gmail.com with ESMTPSA id a20sm4368828lfl.20.2020.07.12.20.51.43 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 12 Jul 2020 20:51:43 -0700 (PDT) Received: by mail-lf1-f48.google.com with SMTP id k15so7439858lfc.4 for ; Sun, 12 Jul 2020 20:51:43 -0700 (PDT) X-Received: by 2002:ac2:5a5e:: with SMTP id r30mr51926816lfn.30.1594612302784; Sun, 12 Jul 2020 20:51:42 -0700 (PDT) MIME-Version: 1.0 References: <20200712215041.GA3644504@google.com> <20200713025354.GB3644504@google.com> In-Reply-To: <20200713025354.GB3644504@google.com> From: Linus Torvalds Date: Sun, 12 Jul 2020 20:51:26 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: WARNING: at mm/mremap.c:211 move_page_tables in i386 To: Joel Fernandes Cc: Naresh Kamboju , linux- stable , open list , linux-mm , Arnd Bergmann , Andrew Morton , Roman Gushchin , Michal Hocko , lkft-triage@lists.linaro.org, Chris Down , Michel Lespinasse , Fan Yang , Brian Geffon , Anshuman Khandual , Will Deacon , Catalin Marinas , pugaowei@gmail.com, Jerome Glisse , Greg Kroah-Hartman , Mel Gorman , Hugh Dickins , Al Viro , Tejun Heo , Sasha Levin Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 12, 2020 at 7:53 PM Joel Fernandes 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