Re: [PATCH] riscv: mm: Enable PMD split page table lock for RV64

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2021/8/5 19:51, Alex Ghiti wrote:
> Hi Kefeng,
>
> On 30/07/2021 15:11, Kefeng Wang wrote:
>> After commit 1355c31eeb7e ("asm-generic: pgalloc: provide generic
>> pmd_alloc_one() and pmd_free_one()"), it is easy to enable
>> ARCH_ENABLE_SPLIT_PMD_PTLOCK for RV64.
>>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   arch/riscv/Kconfig   | 1 +
>>   arch/riscv/mm/init.c | 3 ++-
>>   2 files changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 8fcceb8eda07..c3e456fc7108 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -12,6 +12,7 @@ config 32BIT
>>     config RISCV
>>       def_bool y
>> +    select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
>>       select ARCH_CLOCKSOURCE_INIT
>>       select ARCH_SUPPORTS_ATOMIC_RMW
>>       select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index e547e53cddd2..ce12905a0224 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -361,7 +361,8 @@ static phys_addr_t __init 
>> alloc_pmd_late(uintptr_t va)
>>       unsigned long vaddr;
>>         vaddr = __get_free_page(GFP_KERNEL);
>> -    BUG_ON(!vaddr);
>> +    BUG_ON(!vaddr || !pgtable_pmd_page_ctor(virt_to_page(vaddr)));
>> +
>>       return __pa(vaddr);
>>   }
>>
>
> I took the time to check how the page table lock works, and indeed, 
> the commit you mention makes it easy to enable this feature since the 
> initialization of the lock for the general case is done there and your 
> patch seems correct as this is the only place we allocate a PMD page, 
> and the PTE page allocation already calls pgtable_pte_page_ctor.
>
> Maybe your commit message would need some of the explanation I gave 
> above, or something similar, but this is up to you.
There is a  Documentation/vm/split_page_table_lock.rst doc about this 
feature, we could learn more from changelog with the feature introduced.
>
> You can add:
>
> Reviewed-by: Alexandre Ghiti <alex@ghiti.fr>
Thanks, Alex ;)
>
> Thanks,
>
> Alex
> .
>

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