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Date:   Wed, 24 May 2023 10:42:13 +0200
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Subject: Re: [patch] mm, debug: allow suppressing panic on CONFIG_DEBUG_VM
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To:     Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, Alex Shi <alexs@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Alexander Duyck <alexanderduyck@fb.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        "Justin M. Forbes" <jforbes@fedoraproject.org>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
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On 23.05.23 03:47, Linus Torvalds wrote:
> On Mon, May 22, 2023 at 5:52 PM David Rientjes <rientjes@google.com> wrote:
>>
>> Right now kernel.panic_on_warn can either be 0 or 1.  We can keep the
>> lowest bit to be "panic on all warnings" and then bit-1 as "panic on debug
>> VM warnings."  When CONFIG_DEBUG_VM is enabled, set the new bit by
>> default so there's no behavior change.
> 
> So right now CONFIG_DEBUG_VM being off means that there's nothing at
> all - not just no output, but also no code generation.
> 
> I don't think CONFIG_DEBUG_VM in itself should enable that bit-1 behavior.
> 
> That may be what *you* as a VM person wants, but VM people are not
> exactly the common case.
> 
> So I think we've got several cases:
> 
>   (a) the "don't even build it" case (CONFIG_DEBUG_VM being off)
> 
>   (b) the "build it, and it is a WARN_ON_ONCE()" case
> 
>   (c) the *normal* "panic_on_warn=1" case, which by default would panic
> on all warnings, including any warnings from CONFIG_DEBUG_VM
> 
>   (d) the "VM person" case, which might not panic on normal warnings,
> but would panic on the VM warnings.
> 
> and I think the use-cases are for different classes of kernel use:
> 
>   (a) is for people who disable debugging code until they feel it is
> needed (which I think covers a lot of kernel developers - I certainly
> personally tend to not build with debug support unless I'm chasing
> some issue down)
> 
>   (b) would probably be most distros - enable the warning so that the
> distro can report it, but try not to kill the machine of random people
> 
>   (c) would be most cloud use cases, presumably together with reboot-on-panic
> 
>   (d) would be people who are actual VM developers, and basically want
> the *current* behavior of VM_BUG_ON() with a machine that stops
> 
> and I think (d) is the smallest set of cases of all, but is the one
> you're personally interested in.

Just as a side note, I stumbled yesterday over [1], which apparently 
disables CONFIG_DEBUG_VM on !debug Fedora builds.

The commit description does not contain a rational ( it's empty :) ), 
and I don't know if this is just a temporary change.

I'll CC Justin, maybe Fedora also would like to keep building with 
CONFIG_DEBUG_VM, but default to WARN_ON_ONCE() instead.


[1] 
https://gitlab.com/cki-project/kernel-ark/-/commit/ade780e10ae1fdcb575ab100bf02d61eb12dd406

-- 
Thanks,

David / dhildenb