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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id 34sm12796326qtq.59.2019.06.19.13.13.40 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 19 Jun 2019 13:13:40 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hdgxg-0002nE-42; Wed, 19 Jun 2019 17:13:40 -0300 Date: Wed, 19 Jun 2019 17:13:40 -0300 From: Jason Gunthorpe To: Daniel Vetter Cc: Jerome Glisse , Michal Hocko , Daniel Vetter , Intel Graphics Development , LKML , DRI Development , Linux MM , David Rientjes , Paolo Bonzini , Andrew Morton , Christian =?utf-8?B?S8O2bmln?= Subject: Re: [PATCH 1/4] mm: Check if mmu notifier callbacks are allowed to fail Message-ID: <20190619201340.GL9360@ziepe.ca> References: <20190520213945.17046-1-daniel.vetter@ffwll.ch> <20190521154411.GD3836@redhat.com> <20190618152215.GG12905@phenom.ffwll.local> <20190619165055.GI9360@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) 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 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. Jason