Re: [PATCH] arch: metag: kernel: dma.c: check 'pud' whether is NULL in dma_alloc_init()

[Xishi Qiu <qiuxishi-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2013/11/14 18:24, James Hogan wrote:

> On 14/11/13 09:35, Chen Gang wrote:
>> On 11/14/2013 05:18 PM, James Hogan wrote:
>>> On Thursday 14 November 2013 16:11:21 Chen Gang wrote:
>>>> Like another p?d_alloc(), pud_alloc() also may fail, so need check it.
>>>>
>>>> Signed-off-by: Chen Gang <gang.chen-bOixZGp5f+dBDgjK7y7TUQ@public.gmane.org>
>>>
>>> NAK.
>>>
>>> pud_alloc folds to pud_offset on Meta so it cannot fail.
>>>
>>
>> If so, can pmd_alloc() be fail?
> 
> No. pmd_alloc folds to pmd_offset so it also cannot fail, and the
> existing !pmd check is dead code. The following would be better.
> 
> Cheers
> James
> 

Hi James,

__PAGETABLE_PUD_FOLDED and __PAGETABLE_PMD_FOLDED are defined,
so __pud_alloc() and __pmd_alloc() always return 0, 
and pud_offset() just return (pud_t *)pgd, pmd_offset() just return (pmd_t *)pud,
is that right?

Thanks,
Xishi Qiu

>>From 85a386a9c7df666b1f438435be8a89581bc7e8b3 Mon Sep 17 00:00:00 2001
> From: James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>
> Date: Thu, 14 Nov 2013 10:14:37 +0000
> Subject: [PATCH 1/1] metag: dma: remove dead code in dma_alloc_init()
> 
> Meta has 2 levels of page table so the pmd folds into the pud which
> folds into the pgd. Therefore the !pmd check in dma_alloc_init() is dead
> code since it essentially checks whether:
>   (init_mm->pgd + 0x770) == 0
> 
> Remove the check.
> 
> Reported-by: Chen Gang <gang.chen-bOixZGp5f+dBDgjK7y7TUQ@public.gmane.org>
> Signed-off-by: James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>
> ---
>  arch/metag/kernel/dma.c | 5 -----
>  1 file changed, 5 deletions(-)
> 
> diff --git a/arch/metag/kernel/dma.c b/arch/metag/kernel/dma.c
> index 8c00dedadc54..78205dbc2b86 100644
> --- a/arch/metag/kernel/dma.c
> +++ b/arch/metag/kernel/dma.c
> @@ -401,11 +401,6 @@ static int __init dma_alloc_init(void)
>  		pgd = pgd_offset(&init_mm, CONSISTENT_START);
>  		pud = pud_alloc(&init_mm, pgd, CONSISTENT_START);
>  		pmd = pmd_alloc(&init_mm, pud, CONSISTENT_START);
> -		if (!pmd) {
> -			pr_err("%s: no pmd tables\n", __func__);
> -			ret = -ENOMEM;
> -			break;
> -		}
>  		WARN_ON(!pmd_none(*pmd));
>  
>  		pte = pte_alloc_kernel(pmd, CONSISTENT_START);


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