linux-kernel - Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one

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Message-ID: <20251107114455.59111-1-ioworker0@gmail.com>
Date: Fri,  7 Nov 2025 19:44:55 +0800
From: Lance Yang <ioworker0@...il.com>
To: chenhuacai@...ngson.cn
Cc: akpm@...ux-foundation.org,
	arnd@...db.de,
	chenhuacai@...nel.org,
	jack@...e.cz,
	kevin.brodsky@....com,
	linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	david@...nel.org,
	lorenzo.stoakes@...cle.com,
	vishal.moola@...il.com,
	Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*() about HIGHMEM

From: Lance Yang <lance.yang@...ux.dev>


On Fri,  7 Nov 2025 17:59:22 +0800, Huacai Chen wrote:
> __{pgd,p4d,pud,pmd,pte}_alloc_one_*() always allocate pages with GFP
> flag GFP_PGTABLE_KERNEL/GFP_PGTABLE_USER. These two macros are defined
> as follows:
> 
>  #define GFP_PGTABLE_KERNEL	(GFP_KERNEL | __GFP_ZERO)
>  #define GFP_PGTABLE_USER	(GFP_PGTABLE_KERNEL | __GFP_ACCOUNT)
> 
> There is no __GFP_HIGHMEM in them, so we needn't to clear __GFP_HIGHMEM
> explicitly.

Nice cleanup!

> 
> Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
> ---
> Resend because the lines begin with # was eaten by git.
> 
>  include/asm-generic/pgalloc.h | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
> index 3c8ec3bfea44..706e87b43b19 100644
> --- a/include/asm-generic/pgalloc.h
> +++ b/include/asm-generic/pgalloc.h
> @@ -18,8 +18,7 @@
>   */
>  static inline pte_t *__pte_alloc_one_kernel_noprof(struct mm_struct *mm)
>  {
> -	struct ptdesc *ptdesc = pagetable_alloc_noprof(GFP_PGTABLE_KERNEL &
> -			~__GFP_HIGHMEM, 0);
> +	struct ptdesc *ptdesc = pagetable_alloc_noprof(GFP_PGTABLE_KERNEL, 0);

I looked into the history and it seems you are right. This defensive pattern
was likely introduced by Vishal Moola in commit c787ae5[1].

After this cleanup, would it make sense to add a BUILD_BUG_ON() somewhere
to check that __GFP_HIGHMEM is not present in GFP_PGTABLE_KERNEL and
GFP_PGTABLE_USER? This would prevent any future regression ;)

Just a thought ...

[1] https://github.com/torvalds/linux/commit/c787ae5b391496f4f63bc942c18eb9fdee05741f

Cheers,
Lance

>  
>  	if (!ptdesc)
>  		return NULL;
> @@ -172,7 +171,6 @@ static inline pud_t *__pud_alloc_one_noprof(struct mm_struct *mm, unsigned long
>  
>  	if (mm == &init_mm)
>  		gfp = GFP_PGTABLE_KERNEL;
> -	gfp &= ~__GFP_HIGHMEM;
>  
>  	ptdesc = pagetable_alloc_noprof(gfp, 0);
>  	if (!ptdesc)
> @@ -226,7 +224,6 @@ static inline p4d_t *__p4d_alloc_one_noprof(struct mm_struct *mm, unsigned long
>  
>  	if (mm == &init_mm)
>  		gfp = GFP_PGTABLE_KERNEL;
> -	gfp &= ~__GFP_HIGHMEM;
>  
>  	ptdesc = pagetable_alloc_noprof(gfp, 0);
>  	if (!ptdesc)
> @@ -270,7 +267,6 @@ static inline pgd_t *__pgd_alloc_noprof(struct mm_struct *mm, unsigned int order
>  
>  	if (mm == &init_mm)
>  		gfp = GFP_PGTABLE_KERNEL;
> -	gfp &= ~__GFP_HIGHMEM;
>  
>  	ptdesc = pagetable_alloc_noprof(gfp, order);
>  	if (!ptdesc)