linux-kernel - Re: [PATCH v4 10/27] mm/sparse: allow for alternate vmemmap section init at boot

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Message-ID: <20250226180900.GA1042@cmpxchg.org>
Date: Wed, 26 Feb 2025 13:09:00 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Frank van der Linden <fvdl@...gle.com>
Cc: akpm@...ux-foundation.org, muchun.song@...ux.dev, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, yuzhao@...gle.com,
	usamaarif642@...il.com, joao.m.martins@...cle.com,
	roman.gushchin@...ux.dev
Subject: Re: [PATCH v4 10/27] mm/sparse: allow for alternate vmemmap section
 init at boot

On Tue, Feb 18, 2025 at 06:16:38PM +0000, Frank van der Linden wrote:
> @@ -489,6 +489,14 @@ config SPARSEMEM_VMEMMAP
>  	  SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
>  	  pfn_to_page and page_to_pfn operations.  This is the most
>  	  efficient option when sufficient kernel resources are available.
> +
> +config ARCH_WANT_SPARSEMEM_VMEMMAP_PREINIT
> +	bool
> +
> +config SPARSEMEM_VMEMMAP_PREINIT
> +	bool "Early init of sparse memory virtual memmap"
> +	depends on SPARSEMEM_VMEMMAP && ARCH_WANT_SPARSEMEM_VMEMMAP_PREINIT
> +	default y

oldconfig just prompted me on this, but it's not clear to me what it
does. Not even after skimming the changelog of the patch to be honest.

Can you please add a help text that explains the user-visible effects
of the toggle, as well as guidance as to who might care to change it?