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[209.85.220.65]) by mx.google.com with SMTPS id f19sor5606355oto.86.2019.06.19.13.18.56 for (Google Transport Security); Wed, 19 Jun 2019 13:18:56 -0700 (PDT) Received-SPF: neutral (google.com: 209.85.220.65 is neither permitted nor denied by best guess record for domain of daniel.vetter@ffwll.ch) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@ffwll.ch header.s=google header.b=YlyPqPBi; spf=neutral (google.com: 209.85.220.65 is neither permitted nor denied by best guess record for domain of daniel.vetter@ffwll.ch) smtp.mailfrom=daniel.vetter@ffwll.ch DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=OMu1Eynp9KYFQ9eA5SmgvunmkEBDxoIzU+1szZNiCbA=; b=YlyPqPBiPTfkDOdJ5VJIj4DFNeLCKL8DI+SUapLpqcyHkZ3w7UdvlSsXa0Oewx+gIO ls8A9xGbEJDVy7wX+7Wrp2Y2dOkXE2UKcAt2aGPDe8APV+r21JZ69QC/JyzdhFWazDwg T44DzMD+35dNL6M4TaFtbY9aua85SEnMj9gpc= X-Google-Smtp-Source: APXvYqw6bgG2GsUkNVmJ8zuOKu1tc9Z6lf4W2oeXwaheE4AOJs5xOP8C+LQrEQLCr9yBSA9ecTAu4RInlgBAYdXylok= X-Received: by 2002:a05:6830:ce:: with SMTP id x14mr5140545oto.188.1560975536589; Wed, 19 Jun 2019 13:18:56 -0700 (PDT) MIME-Version: 1.0 References: <20190520213945.17046-1-daniel.vetter@ffwll.ch> <20190521154411.GD3836@redhat.com> <20190618152215.GG12905@phenom.ffwll.local> <20190619165055.GI9360@ziepe.ca> <20190619201340.GL9360@ziepe.ca> In-Reply-To: <20190619201340.GL9360@ziepe.ca> From: Daniel Vetter Date: Wed, 19 Jun 2019 22:18:43 +0200 Message-ID: Subject: Re: [PATCH 1/4] mm: Check if mmu notifier callbacks are allowed to fail To: Jason Gunthorpe Cc: Jerome Glisse , Michal Hocko , Daniel Vetter , Intel Graphics Development , LKML , DRI Development , Linux MM , David Rientjes , Paolo Bonzini , Andrew Morton , =?UTF-8?Q?Christian_K=C3=B6nig?= Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jun 19, 2019 at 10:13 PM Jason Gunthorpe wrote: > On Wed, Jun 19, 2019 at 09:57:15PM +0200, Daniel Vetter wrote: > > On Wed, Jun 19, 2019 at 6:50 PM Jason Gunthorpe wrote: > > > On Tue, Jun 18, 2019 at 05:22:15PM +0200, Daniel Vetter wrote: > > > > On Tue, May 21, 2019 at 11:44:11AM -0400, Jerome Glisse wrote: > > > > > On Mon, May 20, 2019 at 11:39:42PM +0200, Daniel Vetter wrote: > > > > > > Just a bit of paranoia, since if we start pushing this deep into > > > > > > callchains it's hard to spot all places where an mmu notifier > > > > > > implementation might fail when it's not allowed to. > > > > > > > > > > > > Inspired by some confusion we had discussing i915 mmu notifiers and > > > > > > whether we could use the newly-introduced return value to handle some > > > > > > corner cases. Until we realized that these are only for when a task > > > > > > has been killed by the oom reaper. > > > > > > > > > > > > An alternative approach would be to split the callback into two > > > > > > versions, one with the int return value, and the other with void > > > > > > return value like in older kernels. But that's a lot more churn for > > > > > > fairly little gain I think. > > > > > > > > > > > > Summary from the m-l discussion on why we want something at warning > > > > > > level: This allows automated tooling in CI to catch bugs without > > > > > > humans having to look at everything. If we just upgrade the existing > > > > > > pr_info to a pr_warn, then we'll have false positives. And as-is, no > > > > > > one will ever spot the problem since it's lost in the massive amounts > > > > > > of overall dmesg noise. > > > > > > > > > > > > v2: Drop the full WARN_ON backtrace in favour of just a pr_warn for > > > > > > the problematic case (Michal Hocko). > > > > > > I disagree with this v2 note, the WARN_ON/WARN will trigger checkers > > > like syzkaller to report a bug, while a random pr_warn probably will > > > not. > > > > > > I do agree the backtrace is not useful here, but we don't have a > > > warn-no-backtrace version.. > > > > > > IMHO, kernel/driver bugs should always be reported by WARN & > > > friends. We never expect to see the print, so why do we care how big > > > it is? > > > > > > Also note that WARN integrates an unlikely() into it so the codegen is > > > automatically a bit more optimal that the if & pr_warn combination. > > > > Where do you make a difference between a WARN without backtrace and a > > pr_warn? They're both dumped at the same log-level ... > > WARN panics the kernel when you set > > /proc/sys/kernel/panic_on_warn > > So auto testing tools can set that and get a clean detection that the > kernel has failed the test in some way. > > Otherwise you are left with frail/ugly grepping of dmesg. Hm right. Anyway, I'm happy to repaint the bikeshed in any color that's desired, if that helps with landing it. WARN_WITHOUT_BACKTRACE might take a bit longer (need to find a bit of time, plus it'll definitely attract more comments). Michal? -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch