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.
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;
_
