From: Dave Hansen <dave.hansen@linux.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@linux.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) {
@@ -1293,7 +1293,7 @@ static int collapse_pmd_page(pmd_t *pmd,
 	/* Queue the page table to be freed after TLB flush */
 	list_add(&page_ptdesc(pmd_page(old_pmd))->pt_list, pgtables);
 
-	if (IS_ENABLED(CONFIG_X86_32) && !SHARED_KERNEL_PMD) {
+	if (IS_ENABLED(CONFIG_X86_32)) {
 		struct page *page;
 
 		/* Update all PGD tables to use the same large page */
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	2025-04-09 12:00:17.128319285 -0700
+++ b/arch/x86/mm/pgtable.c	2025-04-09 12:53:09.217519767 -0700
@@ -97,18 +97,13 @@ 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);
_

On Mon, Apr 14, 2025 at 10:32:35AM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.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@linux.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

On 4/15/25 01:25, Kirill A. Shutemov wrote:
>>  #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?

It does, and it's right at the beginning of the function. Simplifying
the code here also made it _less_ self-documenting so it needs a better
comment too.

I'll tack the attached patch on to the end of the series.