Re: [PATCH v4 01/11] mm: x86, arm64: add arch_has_hw_pte_young()

From: Hillf Danton <hdanton@sina.com>
To: Yu Zhao <yuzhao@google.com>
Cc: Will Deacon <will@kernel.org>, Linux-MM <linux-mm@kvack.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 01/11] mm: x86, arm64: add arch_has_hw_pte_young()
Date: Sun, 10 Oct 2021 16:59:07 +0800	[thread overview]
Message-ID: <20211010085907.1284-1-hdanton@sina.com> (raw)
In-Reply-To: <CAOUHufbysyPZFZDOaGn+FwVojy-krCZR--7dd0qkWqKL+s9S_Q@mail.gmail.com>

On Thu, 19 Aug 2021 15:23:02 -0600 Yu Zhao wrote:
>On Thu, Aug 19, 2021 at 3:19 AM Will Deacon <will@kernel.org> wrote:
>>
>> How accurate does this need to be? Heterogeneous (big/little) systems are
>> very common on arm64, so the existing code enables hardware access flag
>> unconditionally on CPUs that support it, meaning we could end up running
>> on a system where some CPUs have hardware update and others do not.
>>
>> With your change, we only enable hardware access flag if _all_ CPUs support
>> it (and furthermore, we prevent late onlining of CPUs without the feature
>> if was detected at boot). This sacrifices a lot of flexibility, particularly
>> if we end up tackling CPU errata in this area in future, and it's not clear
>> that it's really required for what you're trying to do.
>
>It doesn't need to be accurate but then my question is how helpful it
>is if it's not accurate.

Alternatively to make the issue simpler, spin without arm64 included given
that it will be revisited once MGLRU lands in the mainline tree.

Hillf