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* [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()
@ 2021-02-01 11:43 Miaohe Lin
  2021-02-01 23:33 ` Andrew Morton
  0 siblings, 1 reply; 3+ messages in thread
From: Miaohe Lin @ 2021-02-01 11:43 UTC (permalink / raw)
  To: akpm; +Cc: linux-mm, linux-kernel, linmiaohe

The condition (A && !C && !D) || !A is equivalent to !A || (A && !C && !D)
and can be further simplified to !A || (!C && !D).

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/pgtable-generic.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index 9578db83e312..fa1375f3e3b2 100644
--- 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;
-- 
2.19.1



^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()
  2021-02-01 11:43 [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush() Miaohe Lin
@ 2021-02-01 23:33 ` Andrew Morton
  2021-02-02  2:09   ` Miaohe Lin
  0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2021-02-01 23:33 UTC (permalink / raw)
  To: Miaohe Lin; +Cc: linux-mm, linux-kernel

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.

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));

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.



^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush()
  2021-02-01 23:33 ` Andrew Morton
@ 2021-02-02  2:09   ` Miaohe Lin
  0 siblings, 0 replies; 3+ messages in thread
From: Miaohe Lin @ 2021-02-02  2:09 UTC (permalink / raw)
  To: Andrew Morton; +Cc: linux-mm, linux-kernel

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.
> 
> .
> 


^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2021-02-02  2:10 UTC | newest]

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2021-02-01 11:43 [PATCH] mm: simplify the VM_BUG_ON condition in pmdp_huge_clear_flush() Miaohe Lin
2021-02-01 23:33 ` Andrew Morton
2021-02-02  2:09   ` Miaohe Lin

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