linux-kernel - Re: [PATCH 3/8] x86/mm: Always tell core mm to sync kernel mappings

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Message-ID: <bqinzxzoiz5pbtgcufgi6o4zfmvyj3q7i7mjtwp2b4x5cek3ca@v5qn4mv2y6ay>
Date: Tue, 15 Apr 2025 11:28:27 +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 3/8] x86/mm: Always tell core mm to sync kernel mappings

On Mon, Apr 14, 2025 at 10:32:37AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Each mm_struct has its own copy of the page tables. When core mm code
> makes changes to a copy of the page tables those changes sometimes
> need to be synchronized with other mms' copies of the page tables. But
> when this synchronization actually needs to happen is highly
> architecture and configuration specific.
> 
> In cases where kernel PMDs are shared across processes
> (SHARED_KERNEL_PMD) the core mm does not itself need to do that
> synchronization for kernel PMD changes. The x86 code communicates
> this by clearing the PGTBL_PMD_MODIFIED bit cleared in those
> configs to avoid expensive synchronization.
> 
> The kernel is moving toward never sharing kernel PMDs on 32-bit.
> Prepare for that and make 32-bit PAE always set PGTBL_PMD_MODIFIED,
> even if there is no modification to synchronize. This obviously adds
> some synchronization overhead in cases where the kernel page tables
> are being changed.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
> 
>  b/arch/x86/include/asm/pgtable-3level_types.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff -puN arch/x86/include/asm/pgtable-3level_types.h~always-set-ARCH_PAGE_TABLE_SYNC_MASK arch/x86/include/asm/pgtable-3level_types.h
> --- a/arch/x86/include/asm/pgtable-3level_types.h~always-set-ARCH_PAGE_TABLE_SYNC_MASK	2025-04-09 11:49:40.552916845 -0700
> +++ b/arch/x86/include/asm/pgtable-3level_types.h	2025-04-09 11:49:40.555916955 -0700
> @@ -29,7 +29,7 @@ typedef union {
>  
>  #define SHARED_KERNEL_PMD	(!static_cpu_has(X86_FEATURE_PTI))
>  
> -#define ARCH_PAGE_TABLE_SYNC_MASK	(SHARED_KERNEL_PMD ? 0 : PGTBL_PMD_MODIFIED)
> +#define ARCH_PAGE_TABLE_SYNC_MASK	PGTBL_PMD_MODIFIED

The new definition is the same between pgtable-2level_types.h and
pgtable-3level_types.h.

Move it to the common pgtable_32_types.h.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov