linux-kernel - Re: [PATCHv3 2/4] x86/64/mm: Make SPARSEMEM

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Message-ID: <aCdbOeK3EkVUTGD2@gmail.com>
Date: Fri, 16 May 2025 17:35:21 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Kieran Bingham <kbingham@...nel.org>,
	Michael Roth <michael.roth@....com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Brijesh Singh <brijesh.singh@....com>,
	Sandipan Das <sandipan.das@....com>,
	Juergen Gross <jgross@...e.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-efi@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv3 2/4] x86/64/mm: Make SPARSEMEM_VMEMMAP the only memory
 model


* Dave Hansen <dave.hansen@...el.com> wrote:

> On 5/16/25 07:08, Ingo Molnar wrote:
> >> And is there any value to support !SPARSEMEM_VMEMMAP?
> > Not really IMHO:
> > 
> >   .config.opensuse.default:    CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
> >   .config.ubuntu.localinstall: CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
> >   .config.fedora.generic:      CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
> >   .config.rhel.generic:        CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
> 
> I look at the distro configs all the time as well.
> 
> But let's also not forget that none of these have lockdep turned on,

Not actually true, quite a few distro debug kernel packages have 
lockdep on, and it's often enabled in internal QA rigs as well.

Not so SPARSEMEM_VMEMMAP...

> [...] and we don't want to toss out lockdep support on x86, for 
> example. ;)

That's an apples to oranges comparison:

 - SPARSEMEM_VMEMMAP is always enabled, in standard and debug kernels 
   as well.

 - lockdep is disabled in standard kernels, enabled in debug kernels 
   and very much relied on.

Thanks,

	Ingo