linux-kernel - Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned

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Message-ID: <6731e921-8ebd-4e98-a582-ca87f2a25d16@quicinc.com>
Date: Fri, 3 Jan 2025 10:01:52 +0800
From: Zhenhua Huang <quic_zhenhuah@...cinc.com>
To: Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual
	<anshuman.khandual@....com>
CC: <will@...nel.org>, <ardb@...nel.org>, <ryan.roberts@....com>,
        <mark.rutland@....com>, <joey.gouly@....com>,
        <dave.hansen@...ux.intel.com>, <akpm@...ux-foundation.org>,
        <chenfeiyang@...ngson.cn>, <chenhuacai@...nel.org>,
        <linux-mm@...ck.org>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Tingwei Zhang <quic_tingweiz@...cinc.com>
Subject: Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not
 section aligned



On 2025/1/3 2:58, Catalin Marinas wrote:
> On Fri, Dec 27, 2024 at 01:19:30PM +0530, Anshuman Khandual wrote:
>> On 12/24/24 19:39, Catalin Marinas wrote:
>>> On Tue, Dec 24, 2024 at 05:32:06PM +0800, Zhenhua Huang wrote:
>>>> I have two ideas:
>>>> 1. Completely disable this PMD mapping optimization since there is no
>>>> guarantee we must align 128M memory for hotplug ..
>>>
>>> I'd be in favour of this, at least if CONFIG_MEMORY_HOTPLUG is enabled.
>>> I think the only advantage here is that we don't allocate a full 2MB
>>> block for vmemmap when only plugging in a sub-section.
>>
>> Agreed, that will be the right fix for the problem which can be back ported.
>> We will have to prevent PMD/PUD/CONT mappings for both linear and as well as
>> vmemmap for all non-boot memory sections, that can be hot-unplugged.
> 
> I haven't read your diffs below yet but why not allow PMD mappings of
> the hotplugged memory (not vmemmap)? SUBSECTION_SHIFT is 21, so we can
> have 2MB blocks in a 4KB page configuration.
> 
Oh, Understood. We discussed the topic and I shared the same concern, 
Anshuman actually preferred to wait for your decision.

See discussion:
https://lore.kernel.org/lkml/3aadda40-c973-4703-8ed8-9cf2d3eb70a0@quicinc.com/

I see your favor now and will post V3 based on that.