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

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Message-ID: <20181210132759.GP1286@dhcp22.suse.cz>
Date:   Mon, 10 Dec 2018 14:27:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian König <christian.koenig@....com>,
        David Rientjes <rientjes@...gle.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 1/4] mm: Check if mmu notifier callbacks are allowed to
 fail

On Mon 10-12-18 11:36:38, 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.

OK, fair enough. If this is going to help with testing then I do not
have any objections of course.

> v2: Drop the full WARN_ON backtrace in favour of just a pr_warn for
> the problematic case (Michal Hocko).

Thanks!

> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: "Christian König" <christian.koenig@....com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
> Cc: "Jérôme Glisse" <jglisse@...hat.com>
> Cc: linux-mm@...ck.org
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> ---
>  mm/mmu_notifier.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 5119ff846769..ccc22f21b735 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -190,6 +190,9 @@ 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-" : "");
> +				if (blockable)
> +					pr_warn("%pS callback failure not allowed\n",
> +						mn->ops->invalidate_range_start);
>  				ret = _ret;
>  			}
>  		}
> -- 
> 2.20.0.rc1
> 

-- 
Michal Hocko
SUSE Labs