Re: [PATCH 4/4] LoongArch: Use atomic operation with set_pte and pte_clear function

[Huacai Chen]

Hi, Bibo,

On Thu, Oct 10, 2024 at 11:50 AM Bibo Mao <maobibo@xxxxxxxxxxx> wrote:
>
> For kernel space area on LoongArch system, both two consecutive page
> table entries should be enabled with PAGE_GLOBAL bit. So with function
> set_pte() and pte_clear(), pte buddy entry is checked and set besides
> its own pte entry. However it is not atomic operation to set both two
> pte entries, there is problem with test_vmalloc test case.
>
> With previous patch, all page table entries are set with PAGE_GLOBAL
> bit at beginning. Only its own pte entry need update with function
> set_pte() and pte_clear(), nothing to do with buddy pte entry.
>
> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> ---
>  arch/loongarch/include/asm/pgtable.h | 44 ++++++++++------------------
>  1 file changed, 15 insertions(+), 29 deletions(-)
>
> diff --git a/arch/loongarch/include/asm/pgtable.h b/arch/loongarch/include/asm/pgtable.h
> index 22e3a8f96213..4be3f0dbecda 100644
> --- a/arch/loongarch/include/asm/pgtable.h
> +++ b/arch/loongarch/include/asm/pgtable.h
> @@ -325,40 +325,26 @@ extern void paging_init(void);
>  static inline void set_pte(pte_t *ptep, pte_t pteval)
>  {
>         WRITE_ONCE(*ptep, pteval);
> +}
>
> -       if (pte_val(pteval) & _PAGE_GLOBAL) {
> -               pte_t *buddy = ptep_buddy(ptep);
> -               /*
> -                * Make sure the buddy is global too (if it's !none,
> -                * it better already be global)
> -                */
> -               if (pte_none(ptep_get(buddy))) {
> -#ifdef CONFIG_SMP
> -                       /*
> -                        * For SMP, multiple CPUs can race, so we need
> -                        * to do this atomically.
> -                        */
> -                       __asm__ __volatile__(
> -                       __AMOR "$zero, %[global], %[buddy] \n"
> -                       : [buddy] "+ZB" (buddy->pte)
> -                       : [global] "r" (_PAGE_GLOBAL)
> -                       : "memory");
> -
> -                       DBAR(0b11000); /* o_wrw = 0b11000 */
> -#else /* !CONFIG_SMP */
> -                       WRITE_ONCE(*buddy, __pte(pte_val(ptep_get(buddy)) | _PAGE_GLOBAL));
> -#endif /* CONFIG_SMP */
> -               }
> -       }
> +static inline unsigned long __ptep_get_and_clear(pte_t *ptep)
> +{
> +       return atomic64_fetch_and(_PAGE_GLOBAL, (atomic64_t *)&pte_val(*ptep));
>  }
>
>  static inline void pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>  {
> -       /* Preserve global status for the pair */
> -       if (pte_val(ptep_get(ptep_buddy(ptep))) & _PAGE_GLOBAL)
> -               set_pte(ptep, __pte(_PAGE_GLOBAL));
> -       else
> -               set_pte(ptep, __pte(0));
> +       __ptep_get_and_clear(ptep);
With the first patch, a kernel pte always take _PAGE_GLOBAL, so we
don't need an expensive atomic operation, just
"set_pte(pte_val(ptep_get(ptep)) & _PAGE_GLOBAL)" is OK here. And then
we don't need a custom ptep_get_and_clear().


Huacai

> +}
> +
> +#define __HAVE_ARCH_PTEP_GET_AND_CLEAR
> +static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> +                                       unsigned long addr, pte_t *ptep)
> +{
> +       unsigned long val;
> +
> +       val = __ptep_get_and_clear(ptep);
> +       return __pte(val);
>  }
>
>  #define PGD_T_LOG2     (__builtin_ffs(sizeof(pgd_t)) - 1)
> --
> 2.39.3
>