Re: [PATCH 4/7] maccess: Use a scoped guard for page faults

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/25/26 14:07, Muhammad Usama Anjum wrote:
> On 25/08/2026 11:36 am, David Hildenbrand (Arm) wrote:
>> On 8/24/26 18:04, Muhammad Usama Anjum wrote:
>>> Kernel nofault copy and string paths open-code page-fault disable and
>>> enable around label-based loops, duplicating cleanup on success and
>>> failure.
>>>
>>> Use a page-fault scope guard instead. Leaving the scope now re-enables
>>> page faults on both paths without separate cleanup at the fault label.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
>>> ---
>>>  mm/maccess.c | 53 +++++++++++++++++++++++++---------------------------
>>>  1 file changed, 25 insertions(+), 28 deletions(-)
>>>
>>> diff --git a/mm/maccess.c b/mm/maccess.c
>>> index c59a0e092d24a..f695ceefe6fcc 100644
>>> --- a/mm/maccess.c
>>> +++ b/mm/maccess.c
>>> @@ -38,18 +38,17 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
>>>  	if (!size)
>>>  		return 0;
>>>  
>>> -	pagefault_disable();
>>> -	if (!(align & 7))
>>> -		copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
>>> -	if (!(align & 3))
>>> -		copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
>>> -	if (!(align & 1))
>>> -		copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
>>> -	copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
>>> -	pagefault_enable();
>>> +	scoped_guard(pagefault) {
>>> +		if (!(align & 7))
>>> +			copy_from_kernel_nofault_loop(dst, src, size, u64, Efault);
>>> +		if (!(align & 3))
>>> +			copy_from_kernel_nofault_loop(dst, src, size, u32, Efault);
>>> +		if (!(align & 1))
>>> +			copy_from_kernel_nofault_loop(dst, src, size, u16, Efault);
>>> +		copy_from_kernel_nofault_loop(dst, src, size, u8, Efault);
>>> +	}
>>
>> While I understand what pagefault_disable+pagefault_enable does, it is
>> completely unclear what scoped_guard(pagefault) does.
>>
>> Should this be scoped_guard(disabled_pagefaults) or sth like that?
>>
> 
> The `pagefault` guard is already defined in `include/linux/uaccess.h`:

Ah, sorry, I thought this was added in this series.

It's completely confusing.

	guard(pagefault)();

meaning "disable pagefault", what?! :)

CCin Viktor: I see that the patch that introduced that has no acks from MM. I
don't blame you, because the file is not actually listed under the MM section.

I think this really needs to be cleaned up to clarify that we *disable* pagefaults.

Usama, can you throw in a patch to clean that up? (calling it nofault,
disable_pagefaults or whatever)

-- 
Cheers,

David