Re: [RFC V1 07/16] arm64/mm: Convert READ_ONCE() as p4dp_get() while accessing P4D

["David Hildenbrand (Arm)" <david@kernel.org>]

>>>  
>>> @@ -2258,4 +2258,21 @@ int pmdp_test_and_clear_young(struct vm_area_struct *vma,
>>>  }
>>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_ARCH_HAS_NONLEAF_PMD_YOUNG */
>>>  
>>> +#if CONFIG_PGTABLE_LEVELS > 3
>>> +phys_addr_t pud_offset_phys(p4d_t *p4dp, unsigned long addr)
>>> +{
>>> +	p4d_t p4d = p4dp_get(p4dp);
>>> +
>>> +	BUG_ON(!pgtable_l4_enabled());
>>
>> Heh, while at it, convert that to a VM_WARN_ON_ONCE() or anything else
>> that is not a BUG.
>>
>> I strongly assume CONFIG_DEBUG_VM checks are sufficient.
> 
> There are multiple similar BUG_ON() instances
> 
> arch/arm64/include/asm/pgtable.h:       BUG_ON(!pgtable_l4_enabled());
> arch/arm64/include/asm/pgtable.h:       BUG_ON(!pgtable_l5_enabled());
> 
> arch/arm64/mm/mmu.c:                    BUG_ON(pmd_val(old_pmd) != 0 &&
> arch/arm64/mm/mmu.c:                    BUG_ON(pud_val(old_pud) != 0 &&
> arch/arm64/mm/mmu.c:            BUG_ON(p4d_val(old_p4d) != 0 &&
> 
> Shall we convert all of them as VM_WARN_ON_ONCE() in a separate patch
> as a pre-requisite ?

In MM at least, we fix them up when touching the code, and only
sometimes convert entire files.

As documented in Documentation/process/coding-style.rst, there might be
some exceptions where BUG_ON makes sense (severe data corruption
possible), and some cases would want to use WARN_ON_ONCE() + recovery code.

All of these cases here sound like VM_WARN_ON_ONCE() is more than
sufficient though, so feel free to convert them all in one shot.

-- 
Cheers,

David