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

[James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>]

On 14/11/13 11:46, Xishi Qiu wrote:
> 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?

Yes exactly (although technically __pud_alloc() and __pmd_alloc() would
never get called due to pgd_none() and pud_none() returning 0, but it
makes little difference).

Cheers
James

> 
>> >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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