Re: [PATCH] mm: Move set_pxd_safe() helpers from generic to platform

[David Hildenbrand <david@redhat.com>]

On 24.09.24 07:28, Anshuman Khandual wrote:
> 
> 
> On 9/20/24 14:12, Anshuman Khandual wrote:
>>
>>
>> On 9/20/24 12:09, David Hildenbrand wrote:
>>> On 20.09.24 07:30, Anshuman Khandual wrote:
>>>> set_pxd_safe() helpers that serve a specific purpose for both x86 and riscv
>>>> platforms, do not need to be in the common memory code. Otherwise they just
>>>> unnecessarily make the common API more complicated. This moves the helpers
>>>> from common code to platform instead.
>>>>
>>>> Cc: Paul Walmsley <paul.walmsley@sifive.com>
>>>> Cc: Palmer Dabbelt <palmer@dabbelt.com>
>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>> Cc: Dave Hansen <dave.hansen@linux.intel.com>
>>>> Cc: David Hildenbrand <david@redhat.com>
>>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>> Cc: x86@kernel.org
>>>> Cc: linux-mm@kvack.org
>>>> Cc: linux-riscv@lists.infradead.org
>>>> Cc: linux-kernel@vger.kernel.org
>>>> Suggested-by: David Hildenbrand <david@redhat.com>
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>> ---
>>>>    arch/riscv/include/asm/pgtable.h | 19 ++++++++++++++++
>>>>    arch/x86/include/asm/pgtable.h   | 37 +++++++++++++++++++++++++++++++
>>>>    include/linux/pgtable.h          | 38 --------------------------------
>>>>    3 files changed, 56 insertions(+), 38 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>>>> index 089f3c9f56a3..39ca652c5ebe 100644
>>>> --- a/arch/riscv/include/asm/pgtable.h
>>>> +++ b/arch/riscv/include/asm/pgtable.h
>>>> @@ -957,6 +957,25 @@ void misc_mem_init(void);
>>>>    extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
>>>>    #define ZERO_PAGE(vaddr) (virt_to_page(empty_zero_page))
>>>>    +/*
>>>> + * Use set_p*_safe(), and elide TLB flushing, when confident that *no*
>>>> + * TLB flush will be required as a result of the "set". For example, use
>>>> + * in scenarios where it is known ahead of time that the routine is
>>>> + * setting non-present entries, or re-setting an existing entry to the
>>>> + * same value. Otherwise, use the typical "set" helpers and flush the
>>>> + * TLB.
>>>> + */
>>>> +#define set_p4d_safe(p4dp, p4d) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(p4d_present(*p4dp) && !p4d_same(*p4dp, p4d)); \
>>>> +    set_p4d(p4dp, p4d); \
>>>> +})
>>>> +
>>>> +#define set_pgd_safe(pgdp, pgd) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(pgd_present(*pgdp) && !pgd_same(*pgdp, pgd)); \
>>>> +    set_pgd(pgdp, pgd); \
>>>> +})
>>>>    #endif /* !__ASSEMBLY__ */
>>>>      #endif /* _ASM_RISCV_PGTABLE_H */
>>>> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
>>>> index e39311a89bf4..fefb52bb6b4d 100644
>>>> --- a/arch/x86/include/asm/pgtable.h
>>>> +++ b/arch/x86/include/asm/pgtable.h
>>>> @@ -1701,6 +1701,43 @@ bool arch_is_platform_page(u64 paddr);
>>>>    #define arch_is_platform_page arch_is_platform_page
>>>>    #endif
>>>>    +/*
>>>> + * Use set_p*_safe(), and elide TLB flushing, when confident that *no*
>>>> + * TLB flush will be required as a result of the "set". For example, use
>>>> + * in scenarios where it is known ahead of time that the routine is
>>>> + * setting non-present entries, or re-setting an existing entry to the
>>>> + * same value. Otherwise, use the typical "set" helpers and flush the
>>>> + * TLB.
>>>> + */
>>>> +#define set_pte_safe(ptep, pte) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
>>>> +    set_pte(ptep, pte); \
>>>> +})
>>>> +
>>>> +#define set_pmd_safe(pmdp, pmd) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(pmd_present(*pmdp) && !pmd_same(*pmdp, pmd)); \
>>>> +    set_pmd(pmdp, pmd); \
>>>> +})
>>>> +
>>>> +#define set_pud_safe(pudp, pud) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(pud_present(*pudp) && !pud_same(*pudp, pud)); \
>>>> +    set_pud(pudp, pud); \
>>>> +})
>>>> +
>>>> +#define set_p4d_safe(p4dp, p4d) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(p4d_present(*p4dp) && !p4d_same(*p4dp, p4d)); \
>>>> +    set_p4d(p4dp, p4d); \
>>>> +})
>>>> +
>>>> +#define set_pgd_safe(pgdp, pgd) \
>>>> +({ \
>>>> +    WARN_ON_ONCE(pgd_present(*pgdp) && !pgd_same(*pgdp, pgd)); \
>>>> +    set_pgd(pgdp, pgd); \
>>>> +})
>>>>    #endif    /* __ASSEMBLY__ */
>>>
>>> I'm wondering if we can completely get rid of these, for example via:
>>>
>>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>>> index d8dbeac8b206..bc71c25930bb 100644
>>> --- a/arch/x86/mm/init_64.c
>>> +++ b/arch/x86/mm/init_64.c
>>> @@ -79,10 +79,8 @@ DEFINE_POPULATE(pmd_populate_kernel, pmd, pte, init)
>>>   static inline void set_##type1##_init(type1##_t *arg1,         \
>>>                          type2##_t arg2, bool init)              \
>>>   {                                                              \
>>> -       if (init)                                               \
>>> -               set_##type1##_safe(arg1, arg2);                 \
>>> -       else                                                    \
>>> -               set_##type1(arg1, arg2);                        \
>>> +       WARN_ON_ONCE(init && ##type1##_present(*arg1) && !##type1##_same(*arg1, arg2)); \
>>> +       set_##type1(arg1, arg2);                                \
>>>   }
>>>   
>>>
>>> We might be able to handle the pgd_populate etc part similarly, possibly getting
>>> rid of the pgd_populate_safe etc as well.
>>>
>>> Assuming I don't miss anything important :)
>>
>> Sounds feasible but will just leave that upto the x86 platform folks to
>> change later on, after this patch which just moves these helpers inside
>> the platform code.
>>
>>>
>>> Ideally, we get rid of the macros here and just use inline functions ...
>>>
>>
>> Sure, makes sense. Will change these as inline functions.
>>
>> -#define set_pte_safe(ptep, pte) \
>> -({ \
>> -       WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte)); \
>> -       set_pte(ptep, pte); \
>> -})
>> +static inline void set_pte_safe(pte_t *ptep, pte_t pte)
>> +{
>> +       WARN_ON_ONCE(pte_present(*ptep) && !pte_same(*ptep, pte));
>> +       set_pte(ptep, pte);
>> +}
>>
> 
> This has hit a road block in converting these macros as static inline
> functions as suggested earlier, because pmd/pud/p4d/pgd_same() macros
> are defined in generic header include/linux/pgtable.h, but way after
> <asm/pgtable.h> gets included. I guess then the current patch should
> be left as it is.

Just to clarify: My ideas was to get rid of set_pte_safe() *completely* 
in x86 code, and turn set_pte_init() etc. into inline functions.

But we can do that on top, if Dave is fine with this patch.

-- 
Cheers,

David / dhildenb