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SubjectRe: [PATCH 4/4] arm64/mm: Drop vm_fault_t argument from __do_page_fault()
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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@arm.com>
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: James Morse <james.morse@arm.com>
>> Cc: Andrey Konovalov <andreyknvl@google.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.

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