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Subject: Re: [PATCH v2 01/17] powerpc/mm: Don't BUG() in hugepd_page()
To:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, aneesh.kumar@linux.ibm.com
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <cover.1556258134.git.christophe.leroy@c-s.fr>
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 <87o94lxdxe.fsf@concordia.ellerman.id.au>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <d8e3c5e3-553b-da0a-46d9-b3f555c767d0@c-s.fr>
Date:   Thu, 2 May 2019 14:11:16 +0200
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Le 02/05/2019 à 14:02, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@c-s.fr> writes:
>> Use VM_BUG_ON() instead of BUG_ON(), as those BUG_ON()
>> are not there to catch runtime errors but to catch errors
>> during development cycle only.
> 
> I've dropped this one and the next, because I don't like VM_BUG_ON().
> 
> Why not? Because it's contradictory. It's a condition that's so
> important that we should BUG, but only if the kernel has been built
> specially for debugging.
> 
> I don't really buy the development cycle distinction, it's not like we
> have a rigorous test suite that we run and then we declare everything's
> gold and ship a product. We often don't find bugs until they're hit in
> the wild.
> 
> For example the recent corruption Joel discovered with STRICT_KERNEL_RWX
> could have been caught by a BUG_ON() to check we weren't patching kernel
> text in radix__change_memory_range(), but he wouldn't have been using
> CONFIG_DEBUG_VM. (See 8adddf349fda)
> 
> I know Aneesh disagrees with me on this, so maybe you two can convince
> me otherwise.
> 

I have no strong oppinion about this. In v1, I replaced them with a 
WARN_ON(), and Aneesh suggested to go with VM_BUG_ON() instead.

My main purpose was to reduce the amount of BUG/BUG_ON and I thought 
those were good candidates, but if you prefer keeping the BUG(), that's 
ok for me. Or maybe you prefered v1 alternatives (series at 
https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=98170) ?

Christophe