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 12:31:17 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Will Deacon <will.deacon@....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 Thu, Jun 06, 2019 at 12:27:40PM +0100, Catalin Marinas wrote:
> On Thu, Jun 06, 2019 at 10:24:01AM +0530, Anshuman Khandual wrote:
> > On 06/04/2019 08:26 PM, Catalin Marinas wrote:
> > > On Mon, Jun 03, 2019 at 12:11:25PM +0530, Anshuman Khandual wrote:
> > >> 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.
> 
> Not bothered really. You can leave them as in your proposal (I was just
> seeing the VM_GROWSDOWN check tightly coupled with the expand_stack(),
> it's fine either way).

Personally, I find it clearer as separate statements, so I'd suggest
keeping it as per Anshuman's proposal.

Thanks,
Mark.