Re: [PATCH] arm64: mm: force write fault for atomic RMW instructions

Christoph Lameter (Ampere)
2024-05-13T22:39:52Z

On Thu, 9 May 2024, Anshuman Khandual wrote:

>
>>> Okay, I was about to ask, but is not calling get_user() for all data
>>> read page faults increase the cost for a hot code path in general for
>>> some potential savings for a very specific use case. Not sure if that
>>> is worth the trade-off.
>>
>> The instruction is cache hot since it must be present in the cpu cache for the fault. So the overhead is minimal.
>>
>
> But could not a pagefault_disable()-enable() window prevent concurring
> page faults for the current process thus degrading its performance.

The cpu is already executing a fault handler in kernel space. There cannot
be an additional user space fault since we do not execute that code
currently.