linux-kernel - Re: [PATCH 4/4] arm64/mm: Drop vm_fault_t argument from __do_page

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Date:   Fri, 31 May 2019 14:35:23 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 4/4] arm64/mm: Drop vm_fault_t argument from
 __do_page_fault()



On 05/29/2019 08:41 PM, Mark Rutland wrote:
> On Wed, May 29, 2019 at 06:04:45PM +0530, Anshuman Khandual wrote:
>> __do_page_fault() is over complicated with multiple goto statements. This
>> cleans up code flow and while there drops the vm_fault_t argument.
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: James Morse <james.morse@....com> 
>> Cc: Andrey Konovalov <andreyknvl@...gle.com>
>> ---
>>  arch/arm64/mm/fault.c | 38 ++++++++++++++++----------------------
>>  1 file changed, 16 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 170c71f..a53a30e 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -397,37 +397,31 @@ static void do_bad_area(unsigned long addr, unsigned int esr, struct pt_regs *re
>>  static vm_fault_t __do_page_fault(struct mm_struct *mm, unsigned long addr,
>>  			   unsigned int mm_flags, unsigned long vm_flags)
>>  {
>> -	struct vm_area_struct *vma;
>> -	vm_fault_t fault;
>> +	struct vm_area_struct *vma = find_vma(mm, addr);
>>  
>> -	vma = find_vma(mm, addr);
>> -	fault = VM_FAULT_BADMAP;
>>  	if (unlikely(!vma))
>> -		goto out;
>> -	if (unlikely(vma->vm_start > addr))
>> -		goto check_stack;
>> +		return VM_FAULT_BADMAP;
>>  
>>  	/*
>> -	 * Ok, we have a good vm_area for this memory access, so we can handle
>> -	 * it.
>> +	 * Check if the VMA has got the required permssion with respect
>> +	 * to the access fault here.
>>  	 */
> 
> We already had a perfectly good comment for this check:
> 
> 	/*
> 	 * Check that the permissions on the VMA allow for the fault which
> 	 * occurred.
> 	 */
> 
> ... so please keep that and minimize the diff.

Sure, will keep all the existing comments here.

> 
>> -good_area:
>> +	if (!(vma->vm_flags & vm_flags))
>> +		return VM_FAULT_BADACCESS;
>> +
>>  	/*
>> -	 * Check that the permissions on the VMA allow for the fault which
>> -	 * occurred.
>> +	 * There is a valid VMA for this access. But before proceeding
>> +	 * make sure that it has required flags if there is an attempt
>> +	 * to expand the stack downwards.
>>  	 */
> 
> I think we can drop this comment, given we didn't have it previously.

Okay.

> 
>> -	if (!(vma->vm_flags & vm_flags)) {
>> -		fault = VM_FAULT_BADACCESS;
>> -		goto out;
>> -	}
>> +	if (unlikely(vma->vm_start > addr)) {
>> +		if (!(vma->vm_flags & VM_GROWSDOWN))
>> +			return VM_FAULT_BADMAP;
>>  
>> +		if (expand_stack(vma, addr))
>> +			return VM_FAULT_BADMAP;
> 
> You can drop the line space between these two if statements.

Will do.

> 
>> +	}
>>  	return handle_mm_fault(vma, addr & PAGE_MASK, mm_flags);
>> -
>> -check_stack:
>> -	if (vma->vm_flags & VM_GROWSDOWN && !expand_stack(vma, addr))
>> -		goto good_area;
>> -out:
>> -	return fault;
> 
> We used to check the stack before the checknig the rest of the vm_flags,
> so this changes the precedence of the VM_FAULT_BADMAP and
> VM_FAULT_BADACCESS return codes.
> 
> Please check the stack before checking the other vm_flags.

Though it makes some sense to move VMA permission check earlier in the function as it
is the quicker one but I understand need to maintain the existing code flow in a clean
up patch like this. Will retain the existing flow.