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

[maobibo <maobibo@loongson.cn>]

Huacai,

On 2024/10/12 上午10:16, Huacai Chen wrote:
> Hi, Bibo,
> 
> On Thu, Oct 10, 2024 at 11:50 AM Bibo Mao <maobibo@loongson.cn> 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@loongson.cn>
>> ---
>>   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().
Will use non-atomic method and test again, also will remove customed 
function ptep_get_and_clear().

Regards
Bibo Mao
> 
> 
> 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
>>