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

[Yeoreum Yun]

Hi Dave,

> 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.

Yes.. Sorry for my poor commit message.

> 
> 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.

Yeap. I've tested with your patch and it works for me!
Thanks!

Reviewed-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
Tested-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>

> 
> ---
> 
>  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;
> _


-- 
Sincerely,
Yeoreum Yun