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SubjectRe: [PATCH -next v2] riscv/mm/fault: simplify code for do_page_fault()
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On Sat, 29 Oct 2022 01:47:15 PDT (-0700), tongtiangen@huawei.com wrote:
> To make the code more hierarchical and readable, we fold vma related
> judgments into __do_page_fault(). And to simplify the code, move the
> tsk->thread.bad_cause's setting into bad_area(). No functional change
> intended.
>
> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
> ---
> v1 -> v2:
> Fixed incorrect use of macro VM_FAULT_BADMAP.
>
> arch/riscv/mm/fault.c | 77 +++++++++++++++++++++++--------------------
> 1 file changed, 41 insertions(+), 36 deletions(-)
>
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index d86f7cebd4a7..3fdc2eebdd36 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -85,6 +85,8 @@ static inline void mm_fault_error(struct pt_regs *regs, unsigned long addr, vm_f
>
> static inline void bad_area(struct pt_regs *regs, struct mm_struct *mm, int code, unsigned long addr)
> {
> + current->thread.bad_cause = regs->cause;
> +
> /*
> * Something tried to access memory that isn't in our memory map.
> * Fix it, but check if it's kernel or user first.
> @@ -200,6 +202,38 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
> return false;
> }
>
> +#define VM_FAULT_BADMAP ((__force vm_fault_t)0x010000)
> +#define VM_FAULT_BADACCESS ((__force vm_fault_t)0x020000)

These alias with VM_FAULT_HINDEX_MASK. I just sent along
<https://lore.kernel.org/all/20221203030356.3917-1-palmer@rivosinc.com/T/#t>
to clean that up for the other ports that also do so, I'm going to hold
off on this patch until I see what the comments on that one look like.

Otherwise this looks fine, it might not make 6.2 pending the discussion
over there.

Thanks!

> +
> +static vm_fault_t __do_page_fault(struct mm_struct *mm, unsigned long addr,
> + unsigned int mm_flags, struct pt_regs *regs)
> +{
> + struct vm_area_struct *vma = find_vma(mm, addr);
> +
> + if (unlikely(!vma))
> + return VM_FAULT_BADMAP;
> +
> + if (unlikely(vma->vm_start > addr)) {
> + if (unlikely(!(vma->vm_flags & VM_GROWSDOWN) ||
> + expand_stack(vma, addr)))
> + return VM_FAULT_BADMAP;
> + }
> +
> + /*
> + * Ok, we have a good vm_area for this memory access, so
> + * we can handle it.
> + */
> + if (unlikely(access_error(regs->cause, vma)))
> + return VM_FAULT_BADACCESS;
> +
> + /*
> + * If for any reason at all we could not handle the fault,
> + * make sure we exit gracefully rather than endlessly redo
> + * the fault.
> + */
> + return handle_mm_fault(vma, addr, mm_flags, regs);
> +}
> +
> /*
> * This routine handles page faults. It determines the address and the
> * problem, and then passes it off to one of the appropriate routines.
> @@ -207,7 +241,6 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
> asmlinkage void do_page_fault(struct pt_regs *regs)
> {
> struct task_struct *tsk;
> - struct vm_area_struct *vma;
> struct mm_struct *mm;
> unsigned long addr, cause;
> unsigned int flags = FAULT_FLAG_DEFAULT;
> @@ -280,44 +313,16 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
> flags |= FAULT_FLAG_INSTRUCTION;
> retry:
> mmap_read_lock(mm);
> - vma = find_vma(mm, addr);
> - if (unlikely(!vma)) {
> - tsk->thread.bad_cause = cause;
> - bad_area(regs, mm, code, addr);
> - return;
> - }
> - if (likely(vma->vm_start <= addr))
> - goto good_area;
> - if (unlikely(!(vma->vm_flags & VM_GROWSDOWN))) {
> - tsk->thread.bad_cause = cause;
> - bad_area(regs, mm, code, addr);
> - return;
> - }
> - if (unlikely(expand_stack(vma, addr))) {
> - tsk->thread.bad_cause = cause;
> - bad_area(regs, mm, code, addr);
> - return;
> - }
>
> - /*
> - * Ok, we have a good vm_area for this memory access, so
> - * we can handle it.
> - */
> -good_area:
> - code = SEGV_ACCERR;
> + fault = __do_page_fault(mm, addr, flags, regs);
>
> - if (unlikely(access_error(cause, vma))) {
> - tsk->thread.bad_cause = cause;
> - bad_area(regs, mm, code, addr);
> - return;
> - }
> + if (unlikely(fault & VM_FAULT_BADMAP))
> + return bad_area(regs, mm, code, addr);
>
> - /*
> - * If for any reason at all we could not handle the fault,
> - * make sure we exit gracefully rather than endlessly redo
> - * the fault.
> - */
> - fault = handle_mm_fault(vma, addr, flags, regs);
> + if (unlikely(fault & VM_FAULT_BADACCESS)) {
> + code = SEGV_ACCERR;
> + return bad_area(regs, mm, code, addr);
> + }
>
> /*
> * If we need to retry but a fatal signal is pending, handle the

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