Re: [PATCH v2 1/4] mm/pgtable: Fix continue to preallocate pmds even if failure occurrence

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Tue, Aug 20, 2013 at 04:04:18PM -0700, Andrew Morton wrote:
>On Tue, 20 Aug 2013 14:54:53 +0800 Wanpeng Li <liwanp@linux.vnet.ibm.com> wrote:
>
>> preallocate_pmds will continue to preallocate pmds even if failure
>> occurrence, and then free all the preallocate pmds if there is
>> failure, this patch fix it by stop preallocate if failure occurrence
>> and go to free path.
>>
>> ...
>>
>> --- a/arch/x86/mm/pgtable.c
>> +++ b/arch/x86/mm/pgtable.c
>> @@ -196,21 +196,18 @@ static void free_pmds(pmd_t *pmds[])
>>  static int preallocate_pmds(pmd_t *pmds[])
>>  {
>>  	int i;
>> -	bool failed = false;
>>  
>>  	for(i = 0; i < PREALLOCATED_PMDS; i++) {
>>  		pmd_t *pmd = (pmd_t *)__get_free_page(PGALLOC_GFP);
>>  		if (pmd == NULL)
>> -			failed = true;
>> +			goto err;
>>  		pmds[i] = pmd;
>>  	}
>>  
>> -	if (failed) {
>> -		free_pmds(pmds);
>> -		return -ENOMEM;
>> -	}
>> -
>>  	return 0;
>> +err:
>> +	free_pmds(pmds);
>> +	return -ENOMEM;
>>  }
>

Hi Andrew,

>Nope.  If the error path is taken, free_pmds() will free uninitialised
>items from pmds[], which is a local in pgd_alloc() and contains random
>stack junk.  The kernel will crash.
>
>You could pass an nr_pmds argument to free_pmds(), or zero out the
>remaining items on the error path.  However, although the current code
>is a bit kooky, I don't see that it is harmful in any way.
>

There is a check in free_pmds():

if (pmds[i])
	free_page((unsigned long)pmds[i]);

which will avoid the issue you mentioned.

In addition, the codes in pgd_alloc will skip free pmds if preallocate pmds 
failure which will avoid free pmds twice. Am I miss something? ;-)

Regards,
Wanpeng Li 

>> Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
>
>Ahem.

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