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

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Message-ID: <Z3bh74I1YhGWLU4M@arm.com>
Date: Thu, 2 Jan 2025 18:58:55 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: Zhenhua Huang <quic_zhenhuah@...cinc.com>, 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
Subject: Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not
 section aligned

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.

-- 
Catalin