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

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Message-ID: <87lgqcy5uw.fsf@e105922-lin.cambridge.arm.com>
Date:   Thu, 04 May 2017 16:55:03 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Catalin Marinas <catalin.marinas@....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

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

> 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.

Indeed - I was avoiding changing the function to drop contiguous
hugepage support which follows this hunk.

I've made changes locally based on your suggestion and will post a
revised version after the merge window.

Thanks,
Punit