Re: [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()

[Miaohe Lin <linmiaohe@huawei.com>]

Hi:
On 2021/2/2 7:33, Andrew Morton wrote:
> On Mon, 1 Feb 2021 06:43:19 -0500 Miaohe Lin <linmiaohe@huawei.com> wrote:
> 
>> The condition (A && !C && !D) || !A is equivalent to !A || (A && !C && !D)
>> and can be further simplified to !A || (!C && !D).
>>
>> ..
>>
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -135,8 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>>  {
>>  	pmd_t pmd;
>>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> -	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
>> -			   !pmd_devmap(*pmdp)) || !pmd_present(*pmdp));
>> +	VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
>> +					  !pmd_devmap(*pmdp)));
>>  	pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
>>  	flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>>  	return pmd;
> 
> True, and the resulting code is still readable enough.
> 
> But a problem with such a complex expression is that the developer will
> have trouble figuring out why the BUG actually triggered.
> 

Agree! We can determine which condition is failing through the line number __but__
we can't figure out exactly which one triggered BUG for a complex expression.

> If we had a VM_BUG_ON_PMD() then we could print the pmd's value and
> permit diagnosis from that.  But we don't have such a thing.
> 
> So I suggest that it would be better to have
> 
> 	VM_BUG_ON((pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
> 			   !pmd_devmap(*pmdp)));
> 	VM_BUG_ON(!pmd_present(*pmdp));
> 
> This way, the BUG()'s file-n-line output will tell us more about why the
> kernel went splat.
> 
> 
> I suppose maybe this could be optimized the same way, as
> 
> 	VM_BUG_ON(!pmd_present(*pmdp));
> 	/* Below assumes pmd_present() is true */
> 	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));

This one looks good and provide more information than before. I can send another patch to do this (and feel free to merge into
this one), should I ?

Many thanks.

> 
> Which works because VM_BUG_ON is, depending up Kconfig, either a no-op
> or a noreturn-if-it-triggered.  I'm not sure if I like this trick much though.
> 
> .
>