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

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Message-ID: <rqkfqkkli57fbd5zkj3bwko44kmqqwnfdm766snm26y2so52ss@6it24qxv356q>
Date: Fri, 16 May 2025 17:01:20 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: 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

On Fri, May 16, 2025 at 06:42:03AM -0700, Dave Hansen wrote:
> On 5/16/25 05:33, Kirill A. Shutemov wrote:
> > 5-level paging only supports SPARSEMEM_VMEMMAP. CONFIG_X86_5LEVEL is
> > being phased out, making 5-level paging support mandatory.
> > 
> > Make CONFIG_SPARSEMEM_VMEMMAP mandatory for x86-64 and eliminate
> > any associated conditional statements.
> I think we have ourselves a catch-22 here.
> 
> SPARSEMEM_VMEMMAP was selected because the other sparsemem modes
> couldn't handle a dynamic MAX_PHYS{MEM,ADDR}_BITS introduced by 5-level
> paging. Now you're proposing making it static again, but keeping the
> SPARSEMEM_VMEMMAP dependency.
> 
> If you remove the dynamic MAX_PHYS{MEM,ADDR}_BITS, you should also
> remove the dependency on SPARSEMEM_VMEMMAP. No?

I guess. But how?

And is there any value to support !SPARSEMEM_VMEMMAP?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov