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SubjectRe: [PATCH 4/4] arm64/mm: Drop vm_fault_t argument from __do_page_fault()
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.

> -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.

> - 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.

> + }
> 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.

Otherwise, this looks like a nice cleanup -- the old control flow was
hideous.

Thanks,
Mark.

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