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

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Message-ID: <ec207271-623a-f2c0-0adf-7ecbc47aa99b@gentwo.org>
Date: Mon, 13 May 2024 15:39:46 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Anshuman Khandual <anshuman.khandual@....com>
cc: Yang Shi <yang@...amperecomputing.com>, catalin.marinas@....com, 
    will@...nel.org, scott@...amperecomputing.com, 
    linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: force write fault for atomic RMW
 instructions

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.