Re: [PATCH RFC v3 15/21] x86: mm: skip collapse_pud_page() when CONFIG_X86_DIRECT_GBPAGES disabled

[Dave Hansen <dave.hansen@intel.com>]

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On 9/2/26 04:56, Yeoreum Yun wrote:
> The behaviour of pXd_page() will change with generic compile-time folded
> page tables by disallowing its use and triggering a compile-time error
> when it's used improperly, ensuring that the actual pXd_page() is used
> instead.
> 
> To prepare fot that, skip collapse_pud_page() when
> CONFIG_X86_DIRECT_GBPAGES is disabled.

Nit: this doesn't explain how the change actually fixes anything or what
the specific problem being solved is.

I think you want to say something along the lines of:

	collapse_pud_page() uses pud_page() in a way which will soon
	trigger a compile-time error on configs that have a folded pud.

	The code which will generate that error is actually unreachable
	on those configs because 'direct_gbpages' is always 0 there.
	However, the compiler does not know that because
	'direct_gbpages' is a normal integer from a separate compilation
	unit.

	Make the compiler aware when most of collapse_pud_page() is
	unreachable by adding a Kconfig check. This ensures it will not
	trip the errors when they are introduced. It probably also trims
	the kernel image down a wee bit too as a side benefit.

Maybe I should just merge something like the attached patch. I think it
would solve your problem and make things generally cleaner too.

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There is an existing variable (direct_gbpages) that says whether the
kernel can and should use 1G pages in the direct map. It is driven
by a bunch of other machinery. At least:

 1. Hardware support for 1G pages
 2. Kconfig support for 1G direct mappings
 3. Kernel command line overrides

Most code just checks the 'direct_gbpages' variable itself. But this
prevents compiler optimization in cases where 1G mappings are
compile-time disabled (via X86_DIRECT_GBPAGES).

Add a helper to replace 'direct_gbpages' checks. Check the Kconfig
option and base CPU support before looking at the variable.

This lets the compiler optimize things better, especially
collapse_pud_page() where most of the function can now be optimized
away.

---

 b/arch/x86/include/asm/pgtable.h     |   14 ++++++++++++++
 b/arch/x86/kernel/cpu/common.c       |    2 +-
 b/arch/x86/kernel/machine_kexec_64.c |    2 +-
 b/arch/x86/mm/init.c                 |    2 +-
 b/arch/x86/mm/pat/set_memory.c       |    4 ++--
 5 files changed, 19 insertions(+), 5 deletions(-)

diff -puN arch/x86/include/asm/pgtable.h~direct_gbpages-compiletime arch/x86/include/asm/pgtable.h
--- a/arch/x86/include/asm/pgtable.h~direct_gbpages-compiletime	2026-09-02 06:53:57.835733398 -0700
+++ b/arch/x86/include/asm/pgtable.h	2026-09-02 08:17:01.434727771 -0700
@@ -1163,6 +1163,20 @@ static inline int pgd_none(pgd_t pgd)
 #ifndef __ASSEMBLER__
 
 extern int direct_gbpages;
+static inline bool direct_gbpages_enabled(void)
+{
+	/* Check the direct map config option: */
+	if (!IS_ENABLED(CONFIG_X86_DIRECT_GBPAGES))
+		return false;
+
+	/* Check the CPU feature: */
+	if (!cpu_feature_enabled(X86_FEATURE_GBPAGES))
+		return false;
+
+	/* Check the command-line and early setup variable: */
+	return direct_gbpages;
+}
+
 void init_mem_mapping(void);
 void early_alloc_pgt_buf(void);
 void __init poking_init(void);
diff -puN arch/x86/mm/init.c~direct_gbpages-compiletime arch/x86/mm/init.c
--- a/arch/x86/mm/init.c~direct_gbpages-compiletime	2026-09-02 06:55:01.004094924 -0700
+++ b/arch/x86/mm/init.c	2026-09-02 08:20:36.759992562 -0700
@@ -251,7 +251,7 @@ static void __init probe_page_size_mask(
 		__default_kernel_pte_mask &= ~_PAGE_GLOBAL;
 
 	/* Enable 1 GB linear kernel mappings if available: */
-	if (direct_gbpages && boot_cpu_has(X86_FEATURE_GBPAGES)) {
+	if (direct_gbpages_enabled()) {
 		printk(KERN_INFO "Using GB pages for direct mapping\n");
 		page_size_mask |= 1 << PG_LEVEL_1G;
 	} else {
diff -puN arch/x86/kernel/cpu/common.c~direct_gbpages-compiletime arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~direct_gbpages-compiletime	2026-09-02 06:57:47.935849397 -0700
+++ b/arch/x86/kernel/cpu/common.c	2026-09-02 06:57:57.299617535 -0700
@@ -2660,7 +2660,7 @@ void __init arch_cpu_finalize_init(void)
 		 * Right now we don't do that with gbpages because there seems
 		 * very little benefit for that case.
 		 */
-		if (!direct_gbpages)
+		if (!direct_gbpages_enabled())
 			set_memory_4k((unsigned long)__va(0), 1);
 	} else {
 		fpu__init_check_bugs();
diff -puN arch/x86/kernel/machine_kexec_64.c~direct_gbpages-compiletime arch/x86/kernel/machine_kexec_64.c
--- a/arch/x86/kernel/machine_kexec_64.c~direct_gbpages-compiletime	2026-09-02 06:57:58.462712975 -0700
+++ b/arch/x86/kernel/machine_kexec_64.c	2026-09-02 06:58:05.219267514 -0700
@@ -257,7 +257,7 @@ static int init_pgtable(struct kimage *i
 		info.kernpg_flag |= _PAGE_ENC;
 	}
 
-	if (direct_gbpages)
+	if (direct_gbpages_enabled())
 		info.direct_gbpages = true;
 
 	for (i = 0; i < nr_pfn_mapped; i++) {
diff -puN arch/x86/mm/pat/set_memory.c~direct_gbpages-compiletime arch/x86/mm/pat/set_memory.c
--- a/arch/x86/mm/pat/set_memory.c~direct_gbpages-compiletime	2026-09-02 06:58:43.518414555 -0700
+++ b/arch/x86/mm/pat/set_memory.c	2026-09-02 06:59:27.252015369 -0700
@@ -130,7 +130,7 @@ void arch_report_meminfo(struct seq_file
 	seq_printf(m, "DirectMap4M:    %8lu kB\n",
 			direct_pages_count[PG_LEVEL_2M] << 12);
 #endif
-	if (direct_gbpages)
+	if (direct_gbpages_enabled())
 		seq_printf(m, "DirectMap1G:    %8lu kB\n",
 			direct_pages_count[PG_LEVEL_1G] << 20);
 }
@@ -1340,7 +1340,7 @@ static int collapse_pud_page(pud_t *pud,
 	pmd_t *pmd, first;
 	int i;
 
-	if (!direct_gbpages)
+	if (!direct_gbpages_enabled())
 		return 0;
 
 	addr &= PUD_MASK;
_