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

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Message-Id: <20250123172432.32468487@davehans-spike.ostc.intel.com>
Date: Thu, 23 Jan 2025 09:24:32 -0800
From: Dave Hansen <dave.hansen@...ux.intel.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org,tglx@...utronix.de,bp@...en8.de,joro@...tes.org,luto@...nel.org,peterz@...radead.org,kirill.shutemov@...ux.intel.com,rick.p.edgecombe@...el.com,jgross@...e.com,Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [RFC][PATCH 2/8] x86/mm: Always "broadcast" PMD setting operations


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 |    2 +-
 b/arch/x86/mm/pgtable.c        |   11 +++--------
 2 files changed, 4 insertions(+), 9 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	2025-01-23 09:20:50.674347666 -0800
+++ b/arch/x86/mm/pat/set_memory.c	2025-01-23 09:20:50.678347810 -0800
@@ -840,7 +840,7 @@ 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) {
diff -puN arch/x86/mm/pgtable.c~always-sync-kernel-mapping-updates arch/x86/mm/pgtable.c
--- a/arch/x86/mm/pgtable.c~always-sync-kernel-mapping-updates	2025-01-23 09:20:50.674347666 -0800
+++ b/arch/x86/mm/pgtable.c	2025-01-23 09:20:50.678347810 -0800
@@ -136,18 +136,13 @@ static void pgd_ctor(struct mm_struct *m
 				KERNEL_PGD_PTRS);
 	}
 
-	/* list required to sync kernel mapping updates */
-	if (!SHARED_KERNEL_PMD) {
-		pgd_set_mm(pgd, mm);
-		pgd_list_add(pgd);
-	}
+	/* List used to sync kernel mapping updates */
+	pgd_set_mm(pgd, mm);
+	pgd_list_add(pgd);
 }
 
 static void pgd_dtor(pgd_t *pgd)
 {
-	if (SHARED_KERNEL_PMD)
-		return;
-
 	spin_lock(&pgd_lock);
 	pgd_list_del(pgd);
 	spin_unlock(&pgd_lock);
_