Re: [PATCH v3 04/26] x86/mm: split out preallocate_sub_pgd()

On Sun, Jul 26, 2026 at 10:22:37PM +0000, Brendan Jackman wrote:
> This code will be needed elsewhere in a following patch. Split out the
> trivial code move for easy review.
> 
> As a side effect, change the logging slightly: instead of directly
> reporting the level of the failure in panic(), show a generic panic
> message, will be preceded by a separate warn that reports the level of
> the failure. This is a simple way to have this helper suit the needs of
> its new user as well as the existing one.
> 
> Other than logging, no functional change intended.
> 
> Signed-off-by: Brendan Jackman <[email protected]>
> ---
>  arch/x86/include/asm/pgalloc.h |  3 +++
>  arch/x86/mm/init_64.c          | 44 +++++++-----------------------------------
>  arch/x86/mm/pgtable.c          | 38 ++++++++++++++++++++++++++++++++++++
>  3 files changed, 48 insertions(+), 37 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h
> index c88691b15f3c6..2aba6cfabf495 100644
> --- a/arch/x86/include/asm/pgalloc.h
> +++ b/arch/x86/include/asm/pgalloc.h
> @@ -2,6 +2,7 @@
>  #ifndef _ASM_X86_PGALLOC_H
>  #define _ASM_X86_PGALLOC_H
>  
> +#include <linux/printk.h>

I think this should go to arch/x86/mm/pgtable.c. pgalloc.h does not need
printk symbols.

>  #include <linux/threads.h>
>  #include <linux/mm.h>		/* for struct page */
>  #include <linux/pagemap.h>
> @@ -128,6 +129,8 @@ static inline void __pud_free_tlb(struct mmu_gather *tlb, pud_t *pud,
>  	___pud_free_tlb(tlb, pud);
>  }
>  
> +extern int preallocate_sub_pgd(struct mm_struct *mm, unsigned long addr);
> +
>  #if CONFIG_PGTABLE_LEVELS > 4
>  static inline void pgd_populate(struct mm_struct *mm, pgd_t *pgd, p4d_t *p4d)
>  {
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index ab4c5a02326f7..ac6688c70872e 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1293,46 +1293,16 @@ static struct kcore_list kcore_vsyscall;
>  static void __init preallocate_vmalloc_pages(void)
>  {
>  	unsigned long addr;
> -	const char *lvl;
>  
>  	for (addr = VMALLOC_START; addr <= VMEMORY_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
> -		pgd_t *pgd = pgd_offset_k(addr);
> -		p4d_t *p4d;
> -		pud_t *pud;
> -
> -		lvl = "p4d";
> -		p4d = p4d_alloc(&init_mm, pgd, addr);
> -		if (!p4d)
> -			goto failed;
> -
> -		if (pgtable_l5_enabled())
> -			continue;
> -
> -		/*
> -		 * The goal here is to allocate all possibly required
> -		 * hardware page tables pointed to by the top hardware
> -		 * level.
> -		 *
> -		 * On 4-level systems, the P4D layer is folded away and
> -		 * the above code does no preallocation.  Below, go down
> -		 * to the pud _software_ level to ensure the second
> -		 * hardware level is allocated on 4-level systems too.
> -		 */
> -		lvl = "pud";
> -		pud = pud_alloc(&init_mm, p4d, addr);
> -		if (!pud)
> -			goto failed;
> +		if (preallocate_sub_pgd(&init_mm, addr)) {
> +			/*
> +			 * The pages have to be there now or they will be
> +			 * missing in process page-tables later.
> +			 */
> +			panic("Failed to pre-allocate pagetables for vmalloc area\n");
> +		}
>  	}
> -
> -	return;
> -
> -failed:
> -
> -	/*
> -	 * The pages have to be there now or they will be missing in
> -	 * process page-tables later.
> -	 */
> -	panic("Failed to pre-allocate %s pages for vmalloc area\n", lvl);
>  }
>  
>  void __init arch_mm_preinit(void)
> diff --git a/arch/x86/mm/pgtable.c b/
> index f32facdb30354..fdd3709509946 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -833,3 +833,41 @@ void arch_check_zapped_pud(struct vm_area_struct *vma, pud_t pud)
>  	/* See note in arch_check_zapped_pte() */
>  	VM_WARN_ON_ONCE(!(vma->vm_flags & VM_SHADOW_STACK) && pud_shstk(pud));
>  }
> +
> +#if CONFIG_PGTABLE_LEVELS > 3
> +/*
> + * Allocate all possibly required hardware page tables pointed to ths
> + * top hardware level. In other words, allocate a p4d on 5-level or a
> + * pud on 4-level.
> + */
> +int preallocate_sub_pgd(struct mm_struct *mm, unsigned long addr)
> +{
> +	const char *lvl;
> +	p4d_t *p4d;
> +	pud_t *pud;
> +
> +	lvl = "p4d";
> +	p4d = p4d_alloc(mm, pgd_offset_pgd(mm->pgd, addr), addr);
> +	if (!p4d)
> +		goto failed;
> +
> +	if (pgtable_l5_enabled())
> +		return 0;
> +
> +	/*
> +	 * On 4-level systems, the P4D layer is folded away and
> +	 * the above code does no preallocation.  Below, go down
> +	 * to the pud _software_ level to ensure the second
> +	 * hardware level is allocated on 4-level systems too.
> +	 */
> +	lvl = "pud";
> +	pud = pud_alloc(mm, p4d, addr);
> +	if (!pud)
> +		goto failed;
> +	return 0;
> +
> +failed:
> +	pr_warn_ratelimited("Failed to preallocate %s\n", lvl);
> +	return -ENOMEM;
> +}
> +#endif
> 
> -- 
> 2.54.0
> 

-- 
Sincerely yours,
Mike.