linux-kernel - Re: [RFC PATCH 1/2] mm/hugetlb: Make huge_pte_offset() consistent between PUD and PMD entries

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Date:   Tue, 25 Jul 2017 15:37:57 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <steve.capper@....com>,
        <will.deacon@....com>, <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [RFC PATCH 1/2] mm/hugetlb: Make huge_pte_offset() consistent between PUD and PMD entries

Catalin Marinas <catalin.marinas@....com> writes:

> Hi Punit,
>
> On Mon, Jul 24, 2017 at 06:33:17PM +0100, Punit Agrawal wrote:
>> When walking the page tables to resolve an address that points to
>> !present_p*d() entry, huge_pte_offset() returns inconsistent values
>> depending on the level of page table (PUD or PMD).
>> 
>> In the case of a PUD entry, it returns NULL while in the case of a PMD
>> entry, it returns a pointer to the page table entry.
>> 
>> Make huge_pte_offset() consistent by always returning NULL on
>> encountering a !present_p*d() entry. Document the behaviour to clarify
>> the expected semantics of this function.
>
> Nitpick: "p*d_present" instead of "present_p*d".

Thanks for spotting. Fixed both the instances locally.

>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index bc48ee783dd9..686eb6fa9eb1 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4603,6 +4603,13 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>>  	return pte;
>>  }
>>  
>> +/*
>> + * huge_pte_offset() - Walk the page table to resolve the hugepage
>> + * entry at address @addr
>> + *
>> + * Return: Pointer to page table entry (PUD or PMD) for address @addr
>> + * or NULL if the entry is not present.
>> + */
>>  pte_t *huge_pte_offset(struct mm_struct *mm,
>>  		       unsigned long addr, unsigned long sz)
>>  {
>> @@ -4617,13 +4624,20 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>>  	p4d = p4d_offset(pgd, addr);
>>  	if (!p4d_present(*p4d))
>>  		return NULL;
>> +
>>  	pud = pud_offset(p4d, addr);
>>  	if (!pud_present(*pud))
>>  		return NULL;
>>  	if (pud_huge(*pud))
>>  		return (pte_t *)pud;
>> +
>>  	pmd = pmd_offset(pud, addr);
>> -	return (pte_t *) pmd;
>> +	if (!pmd_present(*pmd))
>> +		return NULL;
>
> This breaks the current behaviour for swap entries in the pmd (for pud
> is already broken but maybe no-one uses them). It is fixed in the
> subsequent patch together with the pud but the series is no longer
> bisectable. Maybe it's better if you fold the two patches together (or
> change the order, though I'm not sure how readable it is).

I missed the change in behaviour for pmd swap entries. I'll squash the
two patches and re-post.

Thanks for the review.