linux-kernel - Re: [PATCH V2 4/4] arm64/mm: Drop local variable vm_fault_t from __do_page

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Date:   Thu, 6 Jun 2019 10:24:01 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH V2 4/4] arm64/mm: Drop local variable vm_fault_t from
 __do_page_fault()



On 06/04/2019 08:26 PM, Catalin Marinas wrote:
> On Mon, Jun 03, 2019 at 12:11:25PM +0530, Anshuman Khandual wrote:
>> __do_page_fault() is over complicated with multiple goto statements. This
>> cleans up the code flow and while there drops local variable vm_fault_t.
> 
> I'd change the subject as well here to something like refactor or
> simplify __do_page_fault().

Sure.

> 
>> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
>> index 4bb65f3..41fa905 100644
>> --- a/arch/arm64/mm/fault.c
>> +++ b/arch/arm64/mm/fault.c
>> @@ -397,37 +397,29 @@ 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.
>>  	 */
>> -good_area:
>> +	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 could have a single return here:
> 
> 	if (unlikely(vma->vm_start > addr) &&
> 	    (!(vma->vm_flags & VM_GROWSDOWN) || expand_stack(vma, addr)))
> 		return VM_FAULT_BADMAP;
> 
> Not sure it's any clearer though.
> 

TBH the proposed one seems clearer as it separates effect (vma->vm_start > addr)
from required permission check (vma->vm_flags & VM_GROWSDOWN) and required action
(expand_stack(vma, addr)). But I am happy to change as you have mentioned if that
is preferred.