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

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Date:   Fri, 23 Nov 2018 14:15:11 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian König <christian.koenig@....com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 1/3] mm: Check if mmu notifier callbacks are allowed to fail

On Fri, Nov 23, 2018 at 1:43 PM Michal Hocko <mhocko@...nel.org> wrote:
> On Fri 23-11-18 13:30:57, Daniel Vetter wrote:
> > 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.
>
> I wouldn't mind s@...info@...warn@

Well that's too much, because then it would misfire in the oom
testcase, where failing is ok (desireble even, we want to avoid
blocking after all). So needs to be  a switch (or else we need to
filter it in results, and that's a bit a maintenance headache from a
CI pov).
-Danile

> > 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).
>
> Thanks for the clarification.
> --
> Michal Hocko
> SUSE Labs



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch