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

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Message-ID: <a3a60bbb-70b7-49ed-abc6-937e6c13d681@linux.dev>
Date: Wed, 31 Dec 2025 17:52:57 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Wei Yang <richard.weiyang@...il.com>
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 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@...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?

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

> 
> Thanks in advance for your explanation.
> 
>