linux-kernel - Re: [PATCH 2/8] x86/mm: Always "broadcast" PMD setting operations

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Message-ID: <abvsbz4yqc6xe5izuqw25hwj6y7zjhok4tdpp3kqshkuaomhn5@qnykmira2mik>
Date: Tue, 15 Apr 2025 11:25:54 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de, 
	bp@...en8.de, joro@...tes.org, luto@...nel.org, peterz@...radead.org, 
	rick.p.edgecombe@...el.com, jgross@...e.com
Subject: Re: [PATCH 2/8] x86/mm: Always "broadcast" PMD setting operations

On Mon, Apr 14, 2025 at 10:32:35AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Kernel PMDs can either be shared across processes or private to a
> process.  On 64-bit, they are always shared.  32-bit non-PAE hardware
> does not have PMDs, but the kernel logically squishes them into the
> PGD and treats them as private. Here are the four cases:
> 
> 	64-bit:                Shared
> 	32-bit: non-PAE:       Private
> 	32-bit:     PAE+  PTI: Private
> 	32-bit:     PAE+noPTI: Shared
> 
> Note that 32-bit is all "Private" except for PAE+noPTI being an
> oddball.  The 32-bit+PAE+noPTI case will be made like the rest of
> 32-bit shortly.
> 
> But until that can be done, temporarily treat the 32-bit+PAE+noPTI
> case as Private. This will do unnecessary walks across pgd_list and
> unnecessary PTE setting but should be otherwise harmless.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
> 
>  b/arch/x86/mm/pat/set_memory.c |    4 ++--
>  b/arch/x86/mm/pgtable.c        |   11 +++--------
>  2 files changed, 5 insertions(+), 10 deletions(-)
> 
> diff -puN arch/x86/mm/pat/set_memory.c~always-sync-kernel-mapping-updates arch/x86/mm/pat/set_memory.c
> --- a/arch/x86/mm/pat/set_memory.c~always-sync-kernel-mapping-updates	2025-04-09 12:00:17.126319212 -0700
> +++ b/arch/x86/mm/pat/set_memory.c	2025-04-09 12:53:28.082212490 -0700
> @@ -889,7 +889,7 @@ static void __set_pmd_pte(pte_t *kpte, u
>  	/* change init_mm */
>  	set_pte_atomic(kpte, pte);
>  #ifdef CONFIG_X86_32
> -	if (!SHARED_KERNEL_PMD) {
> +	{
>  		struct page *page;
>  
>  		list_for_each_entry(page, &pgd_list, lru) {

Removing the condition, but leaving the block looks sloppy.

Maybe convert #ifdef to IS_ENABLED() while you are there, so it would
justify the block?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov