linux-kernel - Re: [PATCH v9] arm64: mm: Populate vmemmap at the page level if not section aligned

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Message-ID: <Z8a4MWPpTKm2FAce@localhost.localdomain>
Date: Tue, 4 Mar 2025 09:22:09 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Zhenhua Huang <quic_zhenhuah@...cinc.com>
Cc: anshuman.khandual@....com, catalin.marinas@....com, david@...hat.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,
	quic_tingweiz@...cinc.com, stable@...r.kernel.org
Subject: Re: [PATCH v9] arm64: mm: Populate vmemmap at the page level if not
 section aligned

On Tue, Mar 04, 2025 at 03:27:00PM +0800, Zhenhua Huang wrote:
> On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is set
> to 27, making one section 128M. The related page struct which vmemmap
> points to is 2M then.
> Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
> vmemmap to populate at the PMD section level which was suitable
> initially since hot plug granule is always one section(128M). However,
> commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
> existing arm64 assumptions.
> 
> The first problem is that if start or end is not aligned to a section
> boundary, such as when a subsection is hot added, populating the entire
> section is wasteful.
> 
> The next problem is if we hotplug something that spans part of 128 MiB
> section (subsections, let's call it memblock1), and then hotplug something
> that spans another part of a 128 MiB section(subsections, let's call it
> memblock2), and subsequently unplug memblock1, vmemmap_free() will clear
> the entire PMD entry which also supports memblock2 even though memblock2
> is still active.
> 
> Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
> fix similar to x86-64: populate to pages levels if start/end is not aligned
> with section boundary.
> 
> Cc: <stable@...r.kernel.org> # v5.4+
> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
> Acked-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Zhenhua Huang <quic_zhenhuah@...cinc.com>

Reviewed-by: Oscar Salvador <osalvador@...e.de>


-- 
Oscar Salvador
SUSE Labs