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

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Message-ID: <fd90c84b-9829-4699-b7df-c43020519ec9@linux.dev>
Date: Fri, 23 Jan 2026 11:21:50 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Wei Yang <richard.weiyang@...il.com>,
 "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: will@...nel.org, aneesh.kumar@...nel.org, npiggin@...il.com,
 peterz@...radead.org, dev.jain@....com, akpm@...ux-foundation.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 1/22/26 10:00 PM, Wei Yang wrote:
> On Mon, Jan 19, 2026 at 11:18:52AM +0100, David Hildenbrand (Red Hat) wrote:
>> On 1/1/26 03:07, Wei Yang wrote:
>>> 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@...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 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?
>>
>>
>> Do you mean
>>
>> diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
>> index 2faa23d7f8d42..6aeba4bae68d2 100644
>> --- a/mm/mmu_gather.c
>> +++ b/mm/mmu_gather.c
>> @@ -319,7 +319,7 @@ static inline void tlb_table_invalidate(struct mmu_gather
>> *tlb)
>>         }
>> }
>>
>> -#ifdef CONFIG_PT_RECLAIM
>> +#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
>> static inline void __tlb_remove_table_one_rcu(struct rcu_head *head)
>> {
>>         struct ptdesc *ptdesc;
>>
>> ?
> 
> Sorry for the late reply.
> 
> Yes, and maybe we can move the definition to the
> #ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE code block above, then to be next to
> tlb_remove_table_free().
> 
> So that we always have rcu version when CONFIG_MMU_GATHER_RCU_TABLE_FREE.

LGTM, could you help submit an official patch?

Thanks,
Qi

> 
>>
>> -- 
>> Cheers
>>
>> David
>