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 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?

Thanks in advance for your explanation.


-- 
Wei Yang
Help you, Help me