linux-kernel - Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when vma

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Date:   Tue, 15 Feb 2022 10:40:02 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>
CC:     <akpm@...ux-foundation.org>, <naoya.horiguchi@....com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] mm/memory-failure.c: avoid walking page table when
 vma_address() return -EFAULT

On 2022/2/14 22:48, Naoya Horiguchi wrote:
> On Thu, Feb 10, 2022 at 10:17:27PM +0800, Miaohe Lin wrote:
>> It's unnecessary to walk the page table when vma_address() return -EFAULT.
>> Return early if so to save some cpu cycles.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> 
> Acked-by: Naoya Horiguchi <naoya.horiguchi@....com>

Many thanks for your review and Acked-by tag!

> 
> Does this patch fix the real problem rather than just saving cpu cycles?
> Without this patch, "address == -EFAULT" seems to make pgd_offset() return
> invalid pointer and result in some serious result like general protection fault.

I think you're right. We might dereference the invalid pointer in the following pagetable
walk and results in general protection fault.

> If that's the case, this patch might be worth sending to stable.

But I'am not sure vma_address will return -EFAULT for dax pages in the real workload? If so, I will
send a v2 with Fixes tag.

Thanks again.

> 
> Thanks,
> Naoya Horiguchi
> 
>> ---
>>  mm/memory-failure.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index b3ff7e99a421..f86819145ea8 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -315,6 +315,8 @@ static unsigned long dev_pagemap_mapping_shift(struct page *page,
>>  	pmd_t *pmd;
>>  	pte_t *pte;
>>
>> +	if (address == -EFAULT)
>> +		return 0;
>>  	pgd = pgd_offset(vma->vm_mm, address);
>>  	if (!pgd_present(*pgd))
>>  		return 0;
>> ---
>> 2.23.0
>>
> .
>