Re: [PATCH 1/3] mm: Check if mmu notifier callbacks are allowed to fail

[Daniel Vetter]

On Fri, Nov 23, 2018 at 12:15:57PM +0100, Michal Hocko wrote:
> On Thu 22-11-18 17:51:04, 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.
> 
> What does WARN give you more than the existing pr_info? Is really
> backtrace that interesting?

Automated tools have to ignore everything at info level (there's too much
of that). I guess I could do something like

if (blockable)
	pr_warn(...)
else
	pr_info(...)

WARN() is simply my goto tool for getting something at warning level
dumped into dmesg. But I think the pr_warn with the callback function
should be enough indeed.

If you wonder where all the info level stuff happens that we have to
ignore: suspend/resume is a primary culprit (fairly important for
gfx/desktops), but there's a bunch of other places. Even if we ignore
everything at info and below we still need filters because some drivers
are a bit too trigger-happy (i915 definitely included I guess, so everyone
contributes to this problem).

Cheers, Daniel

> 
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: "Christian König" <christian.koenig@xxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > ---
> >  mm/mmu_notifier.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > index 5119ff846769..59e102589a25 100644
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -190,6 +190,8 @@ int __mmu_notifier_invalidate_range_start(struct mm_struct *mm,
> >  				pr_info("%pS callback failed with %d in %sblockable context.\n",
> >  						mn->ops->invalidate_range_start, _ret,
> >  						!blockable ? "non-" : "");
> > +				WARN(blockable,"%pS callback failure not allowed\n",
> > +				     mn->ops->invalidate_range_start);
> >  				ret = _ret;
> >  			}
> >  		}
> > -- 
> > 2.19.1
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch