Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE_FREE

Qi Zheng
2025-12-31T09:53:19Z

On 12/31/25 5:42 PM, Wei Yang wrote:
> On Wed, Dec 17, 2025 at 05:45:48PM +0800, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> The PT_RECLAIM can work on all architectures that support
>> MMU_GATHER_RCU_TABLE_FREE, so make PT_RECLAIM depends on
>> MMU_GATHER_RCU_TABLE_FREE.
>>
>> BTW, change PT_RECLAIM to be enabled by default, since nobody should want
>> to turn it off.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>> arch/x86/Kconfig | 1 -
>> mm/Kconfig | 9 ++-------
>> 2 files changed, 2 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 80527299f859a..0d22da56a71b0 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -331,7 +331,6 @@ config X86
>> select FUNCTION_ALIGNMENT_4B
>> imply IMA_SECURE_AND_OR_TRUSTED_BOOT if EFI
>> select HAVE_DYNAMIC_FTRACE_NO_PATCHABLE
>> - select ARCH_SUPPORTS_PT_RECLAIM if X86_64
>> select ARCH_SUPPORTS_SCHED_SMT if SMP
>> select SCHED_SMT if SMP
>> select ARCH_SUPPORTS_SCHED_CLUSTER if SMP
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index bd0ea5454af82..fc00b429b7129 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -1447,14 +1447,9 @@ config ARCH_HAS_USER_SHADOW_STACK
>> The architecture has hardware support for userspace shadow call
>> stacks (eg, x86 CET, arm64 GCS or RISC-V Zicfiss).
>>
>> -config ARCH_SUPPORTS_PT_RECLAIM
>> - def_bool n
>> -
>> config PT_RECLAIM
>> - bool "reclaim empty user page table pages"
>> - default y
>> - depends on ARCH_SUPPORTS_PT_RECLAIM && MMU && SMP
>> - select MMU_GATHER_RCU_TABLE_FREE
>> + def_bool y
>> + depends on MMU_GATHER_RCU_TABLE_FREE
>> help
>> Try to reclaim empty user page table pages in paths other than munmap
>> and exit_mmap path.
>
> Hi, Qi
>
> I am new to PT_RECLAIM, when reading related code I got one question.
>
> Before this patch, we could have this config combination:
>
> CONFIG_MMU_GATHER_RCU_TABLE_FREE & !CONFIG_PT_RECLAIM
>
> This means tlb_remove_table_free() is rcu version while tlb_remove_table_one()
> is semi rcu version.
>
> I am curious could we use rcu version tlb_remove_table_one() for this case?
> Use rcu version tlb_remove_table_one() if CONFIG_MMU_GATHER_RCU_TABLE_FREE. Is
> there some limitation here?

I think there's no problem. The rcu version can also ensure that the
fast GUP works well.

>
> Thanks in advance for your explanation.
>
>