Re: [PATCH Resend] mm: Refine __{pgd,p4d,pud,pmd,pte}_alloc_one_*() about HIGHMEM

Lance Yang
2025-11-07T11:45:17Z

From: Lance Yang <lance.yang@linux.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@loongson.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)