On Wed, Jun 19, 2019 at 10:13 PM Jason Gunthorpe <jgg@xxxxxxxx> 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 <jgg@xxxxxxxx> 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
