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

[Qi Zheng <qi.zheng@linux.dev>]

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.
> 
>