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

[Muhammad Usama Anjum <usama.anjum@arm.com>]

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`:

  DEFINE_LOCK_GUARD_0(pagefault, pagefault_disable(), pagefault_enable())

Most users use `guard(pagefault)()`, which calls `pagefault_disable()`
in the start and `pagefault_enable()` when the enclosing scope is left.
In case of guard, the scope ends at function exit. scopped_guard() on the
other hand defines the scope with {} (probably called code block).

Use of scoped_guard makes patch 5/7 straightforward:
`scoped_guard(__kernel_nofault_bare)` is nested inside the pagefault
scope.

-- 
Thanks,
Usama