Subject: Re: [PATCH v2 4/7] powerpc: mm: accelerate pagefault when badaccess

From: Michael Ellerman <[email-masked]>
Date: Tue, 09 Apr 2024 18:56:49 +1000

Kefeng Wang <[email-masked]> writes:
> The access_[pkey]_error() of vma already checked under per-VMA lock, if
> it is a bad access, directly handle error, no need to retry with mmap_lock
> again. In order to release the correct lock, pass the mm_struct into
> bad_access_pkey()/bad_access(), if mm is NULL, release vma lock, or
> release mmap_lock. Since the page faut is handled under per-VMA lock,
> count it as a vma lock event with VMA_LOCK_SUCCESS.
>
> Signed-off-by: Kefeng Wang <[email-masked]>
> ---
>  arch/powerpc/mm/fault.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)

I thought there might be a nicer way to do this, plumbing the mm and vma
down through all those levels is a bit of a pain (vma->vm_mm exists
after all).

But I couldn't come up with anything obviously better, without doing
lots of refactoring first, which would be a pain to integrate into this
series.

So anyway, if the series goes ahead:

Acked-by: Michael Ellerman <[email-masked]> (powerpc)

cheers

> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 53335ae21a40..215690452495 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -71,23 +71,26 @@ static noinline int bad_area_nosemaphore(struct pt_regs *regs, unsigned long add
>  	return __bad_area_nosemaphore(regs, address, SEGV_MAPERR);
>  }
>  
> -static int __bad_area(struct pt_regs *regs, unsigned long address, int si_code)
> +static int __bad_area(struct pt_regs *regs, unsigned long address, int si_code,
> +		      struct mm_struct *mm, struct vm_area_struct *vma)
>  {
> -	struct mm_struct *mm = current->mm;
>  
>  	/*
>  	 * Something tried to access memory that isn't in our memory map..
>  	 * Fix it, but check if it's kernel or user first..
>  	 */
> -	mmap_read_unlock(mm);
> +	if (mm)
> +		mmap_read_unlock(mm);
> +	else
> +		vma_end_read(vma);
>  
>  	return __bad_area_nosemaphore(regs, address, si_code);
>  }
>  
>  static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
> +				    struct mm_struct *mm,
>  				    struct vm_area_struct *vma)
>  {
> -	struct mm_struct *mm = current->mm;
>  	int pkey;
>  
>  	/*
> @@ -109,7 +112,10 @@ static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
>  	 */
>  	pkey = vma_pkey(vma);
>  
> -	mmap_read_unlock(mm);
> +	if (mm)
> +		mmap_read_unlock(mm);
> +	else
> +		vma_end_read(vma);
>  
>  	/*
>  	 * If we are in kernel mode, bail out with a SEGV, this will
> @@ -124,9 +130,10 @@ static noinline int bad_access_pkey(struct pt_regs *regs, unsigned long address,
>  	return 0;
>  }
>  
> -static noinline int bad_access(struct pt_regs *regs, unsigned long address)
> +static noinline int bad_access(struct pt_regs *regs, unsigned long address,
> +			       struct mm_struct *mm, struct vm_area_struct *vma)
>  {
> -	return __bad_area(regs, address, SEGV_ACCERR);
> +	return __bad_area(regs, address, SEGV_ACCERR, mm, vma);
>  }
>  
>  static int do_sigbus(struct pt_regs *regs, unsigned long address,
> @@ -479,13 +486,13 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>  
>  	if (unlikely(access_pkey_error(is_write, is_exec,
>  				       (error_code & DSISR_KEYFAULT), vma))) {
> -		vma_end_read(vma);
> -		goto lock_mmap;
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +		return bad_access_pkey(regs, address, NULL, vma);
>  	}
>  
>  	if (unlikely(access_error(is_write, is_exec, vma))) {
> -		vma_end_read(vma);
> -		goto lock_mmap;
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +		return bad_access(regs, address, NULL, vma);
>  	}
>  
>  	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> @@ -521,10 +528,10 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>  
>  	if (unlikely(access_pkey_error(is_write, is_exec,
>  				       (error_code & DSISR_KEYFAULT), vma)))
> -		return bad_access_pkey(regs, address, vma);
> +		return bad_access_pkey(regs, address, mm, vma);
>  
>  	if (unlikely(access_error(is_write, is_exec, vma)))
> -		return bad_access(regs, address);
> +		return bad_access(regs, address, mm, vma);
>  
>  	/*
>  	 * If for any reason at all we couldn't handle the fault,
> -- 
> 2.27.0
>
>
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