Re: [PATCH rfc v2 04/10] s390: mm: use try_vma_locked_page_fault()

[Alexander Gordeev]

On Mon, Aug 21, 2023 at 08:30:50PM +0800, Kefeng Wang wrote:
> Use new try_vma_locked_page_fault() helper to simplify code.
> No functional change intended.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> ---
>  arch/s390/mm/fault.c | 66 ++++++++++++++++++--------------------------
>  1 file changed, 27 insertions(+), 39 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index 099c4824dd8a..fbbdebde6ea7 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -357,16 +357,18 @@ static noinline void do_fault_error(struct pt_regs *regs, vm_fault_t fault)
>  static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  {
>  	struct gmap *gmap;
> -	struct task_struct *tsk;
> -	struct mm_struct *mm;
>  	struct vm_area_struct *vma;
>  	enum fault_type type;
> -	unsigned long address;
> -	unsigned int flags;
> +	struct mm_struct *mm = current->mm;
> +	unsigned long address = get_fault_address(regs);
>  	vm_fault_t fault;
>  	bool is_write;
> +	struct vm_fault vmf = {
> +		.real_address = address,
> +		.flags = FAULT_FLAG_DEFAULT,
> +		.vm_flags = access,
> +	};
>  
> -	tsk = current;
>  	/*
>  	 * The instruction that caused the program check has
>  	 * been nullified. Don't signal single step via SIGTRAP.
> @@ -376,8 +378,6 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	if (kprobe_page_fault(regs, 14))
>  		return 0;
>  
> -	mm = tsk->mm;
> -	address = get_fault_address(regs);
>  	is_write = fault_is_write(regs);
>  
>  	/*
> @@ -398,45 +398,33 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	}
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
> -	flags = FAULT_FLAG_DEFAULT;
>  	if (user_mode(regs))
> -		flags |= FAULT_FLAG_USER;
> +		vmf.flags |= FAULT_FLAG_USER;
>  	if (is_write)
> -		access = VM_WRITE;
> -	if (access == VM_WRITE)
> -		flags |= FAULT_FLAG_WRITE;
> -	if (!(flags & FAULT_FLAG_USER))
> -		goto lock_mmap;
> -	vma = lock_vma_under_rcu(mm, address);
> -	if (!vma)
> -		goto lock_mmap;
> -	if (!(vma->vm_flags & access)) {
> -		vma_end_read(vma);
> -		goto lock_mmap;
> -	}
> -	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> -	if (!(fault & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)))
> -		vma_end_read(vma);
> -	if (!(fault & VM_FAULT_RETRY)) {
> -		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> -		if (likely(!(fault & VM_FAULT_ERROR)))
> -			fault = 0;

This fault fixup is removed in the new version.

> +		vmf.vm_flags = VM_WRITE;
> +	if (vmf.vm_flags == VM_WRITE)
> +		vmf.flags |= FAULT_FLAG_WRITE;
> +
> +	fault = try_vma_locked_page_fault(&vmf);
> +	if (fault == VM_FAULT_NONE)
> +		goto lock_mm;

Because VM_FAULT_NONE is set to 0 it gets confused with
the success code of 0 returned by a fault handler. In the
former case we want to continue, while in the latter -
successfully return. I think it applies to all archs.

> +	if (!(fault & VM_FAULT_RETRY))
>  		goto out;
> -	}
> -	count_vm_vma_lock_event(VMA_LOCK_RETRY);
> +
>  	/* Quick path to respond to signals */
>  	if (fault_signal_pending(fault, regs)) {
>  		fault = VM_FAULT_SIGNAL;
>  		goto out;
>  	}
> -lock_mmap:
> +
> +lock_mm:
>  	mmap_read_lock(mm);
>  
>  	gmap = NULL;
>  	if (IS_ENABLED(CONFIG_PGSTE) && type == GMAP_FAULT) {
>  		gmap = (struct gmap *) S390_lowcore.gmap;
>  		current->thread.gmap_addr = address;
> -		current->thread.gmap_write_flag = !!(flags & FAULT_FLAG_WRITE);
> +		current->thread.gmap_write_flag = !!(vmf.flags & FAULT_FLAG_WRITE);
>  		current->thread.gmap_int_code = regs->int_code & 0xffff;
>  		address = __gmap_translate(gmap, address);
>  		if (address == -EFAULT) {
> @@ -444,7 +432,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  			goto out_up;
>  		}
>  		if (gmap->pfault_enabled)
> -			flags |= FAULT_FLAG_RETRY_NOWAIT;
> +			vmf.flags |= FAULT_FLAG_RETRY_NOWAIT;
>  	}
>  
>  retry:
> @@ -466,7 +454,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	 * we can handle it..
>  	 */
>  	fault = VM_FAULT_BADACCESS;
> -	if (unlikely(!(vma->vm_flags & access)))
> +	if (unlikely(!(vma->vm_flags & vmf.vm_flags)))
>  		goto out_up;
>  
>  	/*
> @@ -474,10 +462,10 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  	 * make sure we exit gracefully rather than endlessly redo
>  	 * the fault.
>  	 */
> -	fault = handle_mm_fault(vma, address, flags, regs);
> +	fault = handle_mm_fault(vma, address, vmf.flags, regs);
>  	if (fault_signal_pending(fault, regs)) {
>  		fault = VM_FAULT_SIGNAL;
> -		if (flags & FAULT_FLAG_RETRY_NOWAIT)
> +		if (vmf.flags & FAULT_FLAG_RETRY_NOWAIT)
>  			goto out_up;
>  		goto out;
>  	}
> @@ -497,7 +485,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  
>  	if (fault & VM_FAULT_RETRY) {
>  		if (IS_ENABLED(CONFIG_PGSTE) && gmap &&
> -			(flags & FAULT_FLAG_RETRY_NOWAIT)) {
> +			(vmf.flags & FAULT_FLAG_RETRY_NOWAIT)) {
>  			/*
>  			 * FAULT_FLAG_RETRY_NOWAIT has been set, mmap_lock has
>  			 * not been released
> @@ -506,8 +494,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>  			fault = VM_FAULT_PFAULT;
>  			goto out_up;
>  		}
> -		flags &= ~FAULT_FLAG_RETRY_NOWAIT;
> -		flags |= FAULT_FLAG_TRIED;
> +		vmf.flags &= ~FAULT_FLAG_RETRY_NOWAIT;
> +		vmf.flags |= FAULT_FLAG_TRIED;
>  		mmap_read_lock(mm);
>  		goto retry;
>  	}

FWIW, this series ends up with kernel BUG at arch/s390/mm/fault.c:341!

Thanks!