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

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Message-ID: <d38fcbe5-8b4a-40bc-b8c8-1c49ccaa9964@kernel.org>
Date: Mon, 19 Jan 2026 11:18:52 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Wei Yang <richard.weiyang@...il.com>, 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,
 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 1/1/26 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;

?

-- 
Cheers

David