Re: [PATCH] mm/hugetlb: avoid unnecessary check on pud and pmd entry in huge_pte_offset

[Mike Kravetz <mike.kravetz@oracle.com>]

Cc a few people who have looked at huge_pte_offset() recently.

On 4/23/20 5:49 AM, Li Xinhai wrote:
> When huge_pte_offset() is called, the parameter sz can only be PUD_SIZE
> or PMD_SIZE.
> If sz is PUD_SIZE and code can reach pud, then *pud must be none, or
> normal hugetlb entry, or non-present (migration or hwpoisoned) hugetlb
> entry, and we can directly return pud.
> When sz is PMD_SIZE, pud must be none or present, and if code can reach
> pmd, we can directly return pmd.
> 
> So, after this patch, the code is simplified by first check on the
> parameter sz, and avoid unnecessary checks in current code.
> 
> Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
>  mm/hugetlb.c | 24 +++++++++---------------
>  1 file changed, 9 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bcabbe0..e1424f5 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5365,8 +5365,8 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  {
>  	pgd_t *pgd;
>  	p4d_t *p4d;
> -	pud_t *pud, pud_entry;
> -	pmd_t *pmd, pmd_entry;
> +	pud_t *pud;
> +	pmd_t *pmd;
>  
>  	pgd = pgd_offset(mm, addr);
>  	if (!pgd_present(*pgd))
> @@ -5376,22 +5376,16 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  		return NULL;
>  
>  	pud = pud_offset(p4d, addr);
> -	pud_entry = READ_ONCE(*pud);
> -	if (sz != PUD_SIZE && pud_none(pud_entry))
> -		return NULL;
> -	/* hugepage or swap? */
> -	if (pud_huge(pud_entry) || !pud_present(pud_entry))
> +	if (sz == PUD_SIZE)
> +		/* must be pud_huge or pud_none */
>  		return (pte_t *)pud;
> -
> -	pmd = pmd_offset(pud, addr);
> -	pmd_entry = READ_ONCE(*pmd);
> -	if (sz != PMD_SIZE && pmd_none(pmd_entry))
> +	if (!pud_present(*pud))
>  		return NULL;
> -	/* hugepage or swap? */
> -	if (pmd_huge(pmd_entry) || !pmd_present(pmd_entry))
> -		return (pte_t *)pmd;
> +	/* must have a valid entry and size to go further */
>  
> -	return NULL;
> +	pmd = pmd_offset(pud, addr);

Can we get here with sz = PMD_SIZE and pud_none(*pud)?  Would that be
an issue for the pmd_offset() call?
-- 
Mike Kravetz
	
> +	/* must be pmd_huge or pmd_none */
> +	return (pte_t *)pmd;
>  }
>  
>  #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
>