[PATCH rfc v2 05/10] powerpc: mm: use try_vma_locked_page_fault()

[Kefeng Wang]

On 2023/8/22 17:38, Christophe Leroy wrote:
> 
> 
> Le 21/08/2023 à 14:30, Kefeng Wang a écrit :
>> Use new try_vma_locked_page_fault() helper to simplify code.
>> No functional change intended.
> 
> Does it really simplifies code ? It's 32 insertions versus 34 deletions
> so only removing 2 lines.

Yes，it is unfriendly for powerpc as the arch's vma access check is much
complex than other arch,
> 
> I don't like the struct vm_fault you are adding because when it was four
> independant variables it was handled through local registers. Now that
> it is a struct it has to go via the stack, leading to unnecessary memory
> read and writes. And going back and forth between architecture code and
> generic code may also be counter-performant.

Because different arch has different var to check vma access, so the
easy way to add them into vmf, I don' find a better way.
> 
> Did you make any performance analysis ? Page faults are really a hot
> path when dealling with minor faults.

no, this is only built and rfc to see the feedback about the conversion.

Thanks.

> 
> Thanks
> Christophe
> 
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang at huawei.com>
>> ---
>>    arch/powerpc/mm/fault.c | 66 ++++++++++++++++++++---------------------
>>    1 file changed, 32 insertions(+), 34 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index b1723094d464..52f9546e020e 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -391,6 +391,22 @@ static int page_fault_is_bad(unsigned long err)
>>    #define page_fault_is_bad(__err)	((__err) & DSISR_BAD_FAULT_32S)
>>    #endif
>>    
>> +#ifdef CONFIG_PER_VMA_LOCK
>> +bool arch_vma_access_error(struct vm_area_struct *vma, struct vm_fault *vmf)
>> +{
>> +	int is_exec = TRAP(vmf->regs) == INTERRUPT_INST_STORAGE;
>> +	int is_write = page_fault_is_write(vmf->fault_code);
>> +
>> +	if (unlikely(access_pkey_error(is_write, is_exec,
>> +				(vmf->fault_code & DSISR_KEYFAULT), vma)))
>> +		return true;
>> +
>> +	if (unlikely(access_error(is_write, is_exec, vma)))
>> +		return true;
>> +	return false;
>> +}
>> +#endif
>> +
>>    /*
>>     * For 600- and 800-family processors, the error_code parameter is DSISR
>>     * for a data fault, SRR1 for an instruction fault.
>> @@ -407,12 +423,18 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>>    {
>>    	struct vm_area_struct * vma;
>>    	struct mm_struct *mm = current->mm;
>> -	unsigned int flags = FAULT_FLAG_DEFAULT;
>>    	int is_exec = TRAP(regs) == INTERRUPT_INST_STORAGE;
>>    	int is_user = user_mode(regs);
>>    	int is_write = page_fault_is_write(error_code);
>>    	vm_fault_t fault, major = 0;
>>    	bool kprobe_fault = kprobe_page_fault(regs, 11);
>> +	struct vm_fault vmf = {
>> +		.real_address = address,
>> +		.fault_code = error_code,
>> +		.regs = regs,
>> +		.flags = FAULT_FLAG_DEFAULT,
>> +	};
>> +
>>    
>>    	if (unlikely(debugger_fault_handler(regs) || kprobe_fault))
>>    		return 0;
>> @@ -463,45 +485,21 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>>    	 * mmap_lock held
>>    	 */
>>    	if (is_user)
>> -		flags |= FAULT_FLAG_USER;
>> +		vmf.flags |= FAULT_FLAG_USER;
>>    	if (is_write)
>> -		flags |= FAULT_FLAG_WRITE;
>> +		vmf.flags |= FAULT_FLAG_WRITE;
>>    	if (is_exec)
>> -		flags |= FAULT_FLAG_INSTRUCTION;
>> +		vmf.flags |= FAULT_FLAG_INSTRUCTION;
>>    
>> -	if (!(flags & FAULT_FLAG_USER))
>> -		goto lock_mmap;
>> -
>> -	vma = lock_vma_under_rcu(mm, address);
>> -	if (!vma)
>> -		goto lock_mmap;
>> -
>> -	if (unlikely(access_pkey_error(is_write, is_exec,
>> -				       (error_code & DSISR_KEYFAULT), vma))) {
>> -		vma_end_read(vma);
>> -		goto lock_mmap;
>> -	}
>> -
>> -	if (unlikely(access_error(is_write, is_exec, vma))) {
>> -		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);
>> +	fault = try_vma_locked_page_fault(&vmf);
>> +	if (fault == VM_FAULT_NONE)
>> +		goto retry;
>> +	if (!(fault & VM_FAULT_RETRY))
>>    		goto done;
>> -	}
>> -	count_vm_vma_lock_event(VMA_LOCK_RETRY);
>>    
>>    	if (fault_signal_pending(fault, regs))
>>    		return user_mode(regs) ? 0 : SIGBUS;
>>    
>> -lock_mmap:
>> -
>>    	/* When running in the kernel we expect faults to occur only to
>>    	 * addresses in user space.  All other faults represent errors in the
>>    	 * kernel and should generate an OOPS.  Unfortunately, in the case of an
>> @@ -528,7 +526,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>>    	 * 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);
>>    
>>    	major |= fault & VM_FAULT_MAJOR;
>>    
>> @@ -544,7 +542,7 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>>    	 * case.
>>    	 */
>>    	if (unlikely(fault & VM_FAULT_RETRY)) {
>> -		flags |= FAULT_FLAG_TRIED;
>> +		vmf.flags |= FAULT_FLAG_TRIED;
>>    		goto retry;
>>    	}
>>