linux-kernel - Re: [PATCH v2 01/17] powerpc/mm: Don't BUG() in hugepd

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Date:   Thu, 02 May 2019 22:02:05 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, aneesh.kumar@...ux.ibm.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 01/17] powerpc/mm: Don't BUG() in hugepd_page()

Christophe Leroy <christophe.leroy@....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.

cheers

> diff --git a/arch/powerpc/include/asm/hugetlb.h b/arch/powerpc/include/asm/hugetlb.h
> index 8d40565ad0c3..7f1867e428c0 100644
> --- a/arch/powerpc/include/asm/hugetlb.h
> +++ b/arch/powerpc/include/asm/hugetlb.h
> @@ -14,7 +14,7 @@
>   */
>  static inline pte_t *hugepd_page(hugepd_t hpd)
>  {
> -	BUG_ON(!hugepd_ok(hpd));
> +	VM_BUG_ON(!hugepd_ok(hpd));
>  	/*
>  	 * We have only four bits to encode, MMU page size
>  	 */
> @@ -42,7 +42,7 @@ static inline void flush_hugetlb_page(struct vm_area_struct *vma,
>  
>  static inline pte_t *hugepd_page(hugepd_t hpd)
>  {
> -	BUG_ON(!hugepd_ok(hpd));
> +	VM_BUG_ON(!hugepd_ok(hpd));
>  #ifdef CONFIG_PPC_8xx
>  	return (pte_t *)__va(hpd_val(hpd) & ~HUGEPD_SHIFT_MASK);
>  #else
> -- 
> 2.13.3