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

[Wei Yang <richard.weiyang@gmail.com>]

On Wed, Dec 31, 2025 at 05:52:57PM +0800, Qi Zheng wrote:
>
>
>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 for your quick response :-)

And Happy New Year

So my little suggestion is move the definition of __tlb_remove_table_one()
under CONFIG_MMU_GATHER_RCU_TABLE_FREE. Do you thinks this would be more
clear?

>> 
>> Thanks in advance for your explanation.
>> 
>> 

-- 
Wei Yang
Help you, Help me