linux-kernel - Re: [PATCH 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries

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Message-ID: <20170503124906.GA5622@e104818-lin.cambridge.arm.com>
Date:   Wed, 3 May 2017 13:49:06 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     will.deacon@....com, David Woods <dwoods@...lanox.com>,
        steve.capper@....com, tbaicar@...eaurora.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] arm64: hugetlb: Fix huge_pte_offset to return
 poisoned page table entries

On Wed, Apr 12, 2017 at 03:04:57PM +0100, Punit Agrawal wrote:
> When memory failure is enabled, a poisoned hugepage pte is marked as a
> swap entry. huge_pte_offset() does not return the poisoned page table
> entries when it encounters PUD/PMD hugepages.
> 
> This behaviour of huge_pte_offset() leads to error such as below when
> munmap is called on poisoned hugepages.
> 
> [  344.165544] mm/pgtable-generic.c:33: bad pmd 000000083af00074.
> 
> Fix huge_pte_offset() to return the poisoned pte which is then
> appropriately handled by the generic layer code.
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Steve Capper <steve.capper@....com>
> Cc: David Woods <dwoods@...lanox.com>
> ---
>  arch/arm64/mm/hugetlbpage.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> index 7514a000e361..5f1832165d69 100644
> --- a/arch/arm64/mm/hugetlbpage.c
> +++ b/arch/arm64/mm/hugetlbpage.c
> @@ -143,15 +143,24 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long 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))
> -		return NULL;
>  
> -	if (pud_huge(*pud))
> +	pud = pud_offset(pgd, addr);
> +	/*
> +	 * In case of HW Poisoning, a hugepage pud/pmd can contain
> +	 * poisoned entries. Poisoned entries are marked as swap
> +	 * entries.
> +	 *
> +	 * For puds/pmds that are not present, check to see if it
> +	 * could be a swap entry (!present and !none).
> +	 */
> +	if ((!pte_present(pud_pte(*pud)) && !pud_none(*pud)) || pud_huge(*pud))
>  		return (pte_t *)pud;

Since we use puds as huge pages, can we just change pud_present() to
match the pmd_present()? I'd like to see similar checks for pud and pmd,
it would be easier to follow. Something like (unchecked):

	if (pud_none(*pud))
		return NULL;
	/* swap or huge page */
	if (!pud_present(*pud) || pud_huge(*pud))
		return (pte_t *)pud;
	/* table; check the next level */

> +
>  	pmd = pmd_offset(pud, addr);
> -	if (!pmd_present(*pmd))
> +	if (pmd_none(*pmd))
>  		return NULL;
> +	if (!pmd_present(*pmd) && !pmd_none(*pmd))
> +		return (pte_t *)pmd;

At this point, we already know that pmd_none(*pmd) is false, no ned to
check it again.

-- 
Catalin