From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Gunthorpe Subject: Re: [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to fail Date: Fri, 16 Aug 2019 14:19:04 -0300 Message-ID: <20190816171904.GA3166@ziepe.ca> References: <20190814202027.18735-1-daniel.vetter@ffwll.ch> <20190814202027.18735-2-daniel.vetter@ffwll.ch> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <20190814202027.18735-2-daniel.vetter@ffwll.ch> Sender: linux-kernel-owner@vger.kernel.org To: Daniel Vetter Cc: LKML , linux-mm@kvack.org, DRI Development , Intel Graphics Development , Andrew Morton , Michal Hocko , Christian =?utf-8?B?S8O2bmln?= , David Rientjes , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Paolo Bonzini , Daniel Vetter List-Id: intel-gfx@lists.freedesktop.org On Wed, Aug 14, 2019 at 10:20:23PM +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). > > v3: Rebase on top of Glisse's arg rework. > > v4: More rebase on top of Glisse reworking everything. > > v5: Fixup rebase damage and also catch failures != EAGAIN for > !blockable (Jason). Also go back to WARN_ON as requested by Jason, so > automatic checkers can easily catch bugs by setting panic_on_warn. > > Cc: Andrew Morton > Cc: Michal Hocko > Cc: "Christian König" > Cc: David Rientjes > Cc: Daniel Vetter > Cc: "Jérôme Glisse" > Cc: linux-mm@kvack.org > Cc: Paolo Bonzini > Cc: Jason Gunthorpe > Signed-off-by: Daniel Vetter > --- > mm/mmu_notifier.c | 2 ++ > 1 file changed, 2 insertions(+) Applied to hmm.git, thanks Jason