Re: [PATCH 1/4] mm: Check if mmu notifier callbacks are allowed to fail

[Jason Gunthorpe]

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.

Jason