Re: [PATCH] mm/hugetlb: LoongArch: Return NULL from huge_pte_offset() for none PMD

[Huacai Chen]

Applied, thanks.

Huacai

On Thu, Apr 24, 2025 at 4:30 PM Ming Wang <wangming01@xxxxxxxxxxx> wrote:
>
> LoongArch's huge_pte_offset() currently returns a pointer to a PMD slot
> even if the underlying entry points to invalid_pte_table (indicating no
> mapping). Callers like smaps_hugetlb_range() fetch this invalid entry
> value (the address of invalid_pte_table) via this pointer.
>
> The generic is_swap_pte() check then incorrectly identifies this address
> as a swap entry on LoongArch, because it satisfies the !pte_present()
> && !pte_none() conditions. This misinterpretation, combined with a
> coincidental match by is_migration_entry() on the address bits, leads
> to kernel crashes in pfn_swap_entry_to_page().
>
> Fix this at the architecture level by modifying huge_pte_offset() to
> check the PMD entry's content using pmd_none() before returning. If the
> entry is none (i.e., it points to invalid_pte_table), return NULL
> instead of the pointer to the slot.
>
> Co-developed-by: Hongchen Zhang <zhanghongchen@xxxxxxxxxxx>
> Signed-off-by: Hongchen Zhang <zhanghongchen@xxxxxxxxxxx>
> Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> Signed-off-by: Ming Wang <wangming01@xxxxxxxxxxx>
> ---
>  arch/loongarch/mm/hugetlbpage.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/mm/hugetlbpage.c b/arch/loongarch/mm/hugetlbpage.c
> index e4068906143b..cea84d7f2b91 100644
> --- a/arch/loongarch/mm/hugetlbpage.c
> +++ b/arch/loongarch/mm/hugetlbpage.c
> @@ -47,7 +47,7 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr,
>                                 pmd = pmd_offset(pud, addr);
>                 }
>         }
> -       return (pte_t *) pmd;
> +       return pmd_none(pmdp_get(pmd)) ? NULL : (pte_t *) pmd;
>  }
>
>  uint64_t pmd_to_entrylo(unsigned long pmd_val)
> --
> 2.43.0
>