Re: [PATCH 2/4] arm64: hugetlbpages: Correctly handle swap entries in huge_pte_offset()

[Mark Rutland]

Hi Punit,

On Thu, Mar 30, 2017 at 05:38:47PM +0100, Punit Agrawal wrote:
> huge_pte_offset() does not correctly handle poisoned or migration page
> table entries. 

What exactly does it do wrong?

Judging by the patch, we return NULL in some cases we shouldn't, right?

What can result from this? e.g. can we see data corruption?

> Not knowing the size of the hugepage entry being
> requested only compounded the problem.
> 
> The recently added hstate parameter can be used to determine the size of
> hugepage being accessed. Use the size to find the correct page table
> entry to return when coming across a swap page table entry.
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
> Cc: David Woods <dwoods@xxxxxxxxxxxx>

Given this is a fix for a bug, it sounds like it should have a fixes
tag, or a Cc stable...

Thanks,
Mark.

> ---
>  arch/arm64/mm/hugetlbpage.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 9ca742c4c1ab..44014403081f 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -192,38 +192,39 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>  pte_t *huge_pte_offset(struct mm_struct *mm,
>  		       unsigned long addr, struct hstate *h)
>  {
> +	unsigned long sz = huge_page_size(h);
>  	pgd_t *pgd;
>  	pud_t *pud;
> -	pmd_t *pmd = NULL;
> -	pte_t *pte = NULL;
> +	pmd_t *pmd;
> +	pte_t *pte;
>  
>  	pgd = pgd_offset(mm, addr);
>  	pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
>  	if (!pgd_present(*pgd))
>  		return NULL;
> +
>  	pud = pud_offset(pgd, addr);
> -	if (!pud_present(*pud))
> +	if (pud_none(*pud) && sz != PUD_SIZE)
>  		return NULL;
> -
> -	if (pud_huge(*pud))
> +	else if (!pud_table(*pud))
>  		return (pte_t *)pud;
> +
> +	if (sz == CONT_PMD_SIZE)
> +		addr &= CONT_PMD_MASK;
> +
>  	pmd = pmd_offset(pud, addr);
> -	if (!pmd_present(*pmd))
> +	if (pmd_none(*pmd) &&
> +	    !(sz == PMD_SIZE || sz == CONT_PMD_SIZE))
>  		return NULL;
> -
> -	if (pte_cont(pmd_pte(*pmd))) {
> -		pmd = pmd_offset(
> -			pud, (addr & CONT_PMD_MASK));
> -		return (pte_t *)pmd;
> -	}
> -	if (pmd_huge(*pmd))
> +	else if (!pmd_table(*pmd))
>  		return (pte_t *)pmd;
> -	pte = pte_offset_kernel(pmd, addr);
> -	if (pte_present(*pte) && pte_cont(*pte)) {
> +
> +	if (sz == CONT_PTE_SIZE) {
>  		pte = pte_offset_kernel(
>  			pmd, (addr & CONT_PTE_MASK));
>  		return pte;
>  	}
> +
>  	return NULL;
>  }
>  
> -- 
> 2.11.0
> 
> 
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