linux-kernel - Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE

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Message-ID: <20251231094243.zmjs7kgflm7q6k73@master>
Date: Wed, 31 Dec 2025 09:42:43 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: will@...nel.org, aneesh.kumar@...nel.org, npiggin@...il.com,
	peterz@...radead.org, dev.jain@....com, akpm@...ux-foundation.org,
	david@...nel.org, ioworker0@...il.com, linmag7@...il.com,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-alpha@...r.kernel.org,
	loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
	linux-parisc@...r.kernel.org, linux-um@...ts.infradead.org,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on
 MMU_GATHER_RCU_TABLE_FREE

On Wed, Dec 17, 2025 at 05:45:48PM +0800, Qi Zheng wrote:
>From: Qi Zheng <zhengqi.arch@...edance.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@...edance.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