Re: [PATCH] mm/pgtable-generic.c: optimize the VM_BUG_ON condition in pmdp_huge_clear_flush()

[Miaohe Lin]

Hi:
On 2021/2/4 20:36, Matthew Wilcox wrote:
> On Wed, Feb 03, 2021 at 03:41:37AM -0500, Miaohe Lin wrote:
>> The developer will have trouble figuring out why the BUG actually triggered
>> when there is a complex expression in the VM_BUG_ON. Because we can only
>> identify the condition triggered BUG via line number provided by VM_BUG_ON.
>> Optimize this by spliting such a complex expression into two simple
>> conditions.
> 
>>  	pmd_t pmd;
>>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>> -	VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) &&
>> -					  !pmd_devmap(*pmdp)));
>> +	VM_BUG_ON(!pmd_present(*pmdp));
>> +	/* Below assumes pmd_present() is true */
>> +	VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
> 
> This is not a complex condition.  We're in the huge PMD handling case
> and we're looking at a PMD which either isn't present or isn't huge.
> It might be useful to print out the PMD in such a case, but splitting
> this into the two cases of pmd-not-present and pmd-isn't-huge isn't
> particularly useful.
> 

Many thanks for your time. You are right that it would be more helpful if we
had a VM_BUG_ON_PMD() that we could print the pmd's value and permit diagnosis
from that. I think splitting this into the two cases is the best we can do now
while lacking of such helper.

> I think you know that, or you wouldn't feel the need to put in a
> comment explaining it!
> .
> 

Thanks again.