Received-SPF: pass (google.com: domain of linux-kernel+bounces-178146-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Mon, 13 May 2024 15:39:46 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@gentwo.org>
To: Anshuman Khandual <anshuman.khandual@arm.com>
cc: Yang Shi <yang@os.amperecomputing.com>, catalin.marinas@arm.com, 
    will@kernel.org, scott@os.amperecomputing.com, 
    linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: mm: force write fault for atomic RMW
 instructions
In-Reply-To: <ea88e906-3219-48c0-884c-3bb13ca0d18f@arm.com>
Message-ID: <ec207271-623a-f2c0-0adf-7ecbc47aa99b@gentwo.org>
References: <20240507223558.3039562-1-yang@os.amperecomputing.com> <c9c3c0d5-bb61-4eed-8b89-b0341a2c6f5c@arm.com> <c1ba9ba3-b0d6-4c6c-d628-614751d737c2@gentwo.org> <ea88e906-3219-48c0-884c-3bb13ca0d18f@arm.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed

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.