Subject: Re: [PATCH 05/10] powerpc/hugetlb: Split the function 'huge_pte_alloc'

From: Anshuman Khandual <[email-masked]>
Date: Wed, 13 Apr 2016 16:38:38 +0530

On 04/11/2016 07:21 PM, Balbir Singh wrote:
> 
> 
> On 07/04/16 15:37, Anshuman Khandual wrote:
>> Currently the function 'huge_pte_alloc' has got two versions, one for the
>> BOOK3S server and the other one for the BOOK3E embedded platforms. This
>> change splits only the BOOK3S server version into two parts, one for the
>> ARCH_WANT_GENERAL_HUGETLB config implementation and the other one for
>> everything else. This change is one of the prerequisites towards enabling
>> ARCH_WANT_GENERAL_HUGETLB config option on POWER platform.
>>
>> Signed-off-by: Anshuman Khandual <[email-masked]>
>> ---
>>  arch/powerpc/mm/hugetlbpage.c | 67 +++++++++++++++++++++++++++----------------
>>  1 file changed, 43 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
>> index d991b9e..e453918 100644
>> --- a/arch/powerpc/mm/hugetlbpage.c
>> +++ b/arch/powerpc/mm/hugetlbpage.c
>> @@ -59,6 +59,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
>>  	return __find_linux_pte_or_hugepte(mm->pgd, addr, NULL, NULL);
>>  }
>>  
>> +#ifndef CONFIG_ARCH_WANT_GENERAL_HUGETLB
>>  static int __hugepte_alloc(struct mm_struct *mm, hugepd_t *hpdp,
>>  			   unsigned long address, unsigned pdshift, unsigned pshift)
>>  {
>> @@ -116,6 +117,7 @@ static int __hugepte_alloc(struct mm_struct *mm, hugepd_t *hpdp,
>>  	spin_unlock(&mm->page_table_lock);
>>  	return 0;
>>  }
>> +#endif /* !CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>>  
>>  /*
>>   * These macros define how to determine which level of the page table holds
>> @@ -130,6 +132,7 @@ static int __hugepte_alloc(struct mm_struct *mm, hugepd_t *hpdp,
>>  #endif
>>  
>>  #ifdef CONFIG_PPC_BOOK3S_64
>> +#ifndef CONFIG_ARCH_WANT_GENERAL_HUGETLB
>>  /*
>>   * At this point we do the placement change only for BOOK3S 64. This would
>>   * possibly work on other subarchs.
>> @@ -145,32 +148,23 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, unsigned long addr, unsigned long sz
>>  
>>  	addr &= ~(sz-1);
>>  	pg = pgd_offset(mm, addr);
>> -
>> -	if (pshift == PGDIR_SHIFT)
>> -		/* 16GB huge page */
>> -		return (pte_t *) pg;
>> -	else if (pshift > PUD_SHIFT)
>> -		/*
>> -		 * We need to use hugepd table
>> -		 */
>> +	if (pshift > PUD_SHIFT) {
>>  		hpdp = (hugepd_t *)pg;
>> -	else {
>> -		pdshift = PUD_SHIFT;
>> -		pu = pud_alloc(mm, pg, addr);
>> -		if (pshift == PUD_SHIFT)
>> -			return (pte_t *)pu;
>> -		else if (pshift > PMD_SHIFT)
>> -			hpdp = (hugepd_t *)pu;
>> -		else {
>> -			pdshift = PMD_SHIFT;
>> -			pm = pmd_alloc(mm, pu, addr);
>> -			if (pshift == PMD_SHIFT)
>> -				/* 16MB hugepage */
>> -				return (pte_t *)pm;
>> -			else
>> -				hpdp = (hugepd_t *)pm;
>> -		}
>> +		goto hugepd_search;
>>  	}
>> +
>> +	pdshift = PUD_SHIFT;
>> +	pu = pud_alloc(mm, pg, addr);
>> +	if (pshift > PMD_SHIFT) {
>> +		hpdp = (hugepd_t *)pu;
>> +		goto hugepd_search;
>> +	}
>> +
>> +	pdshift = PMD_SHIFT;
>> +	pm = pmd_alloc(mm, pu, addr);
>> +	hpdp = (hugepd_t *)pm;
>> +
>> +hugepd_search:
>>  	if (!hpdp)
>>  		return NULL;
>>  
>> @@ -182,6 +176,31 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, unsigned long addr, unsigned long sz
>>  	return hugepte_offset(*hpdp, addr, pdshift);
>>  }
>>  
>> +#else /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>> +pte_t *huge_pte_alloc(struct mm_struct *mm, unsigned long addr, unsigned long sz)
> 
> This is confusing, aren't we using the one from mm/hugetlb.c?

We are using huge_pte_alloc() from mm/hugetlb.c only when we have
CONFIG_ARCH_WANT_GENERAL_HUGETLB enabled. For every thing else we
use the definition here for BOOK3S platforms.

> 
>> +{
>> +	pgd_t *pg;
>> +	pud_t *pu;
>> +	pmd_t *pm;
>> +	unsigned pshift = __ffs(sz);
>> +
>> +	addr &= ~(sz-1);
> 
> Am I reading this right? Shouldn't this be addr &= ~(1 << pshift - 1)

Both are same. __ffs() computes the __ilog2 of the size and arrives at
the page shift. Here we use the size directly instead.