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

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Message-ID: <20260124014552.um257bwrhwjktdca@master>
Date: Sat, 24 Jan 2026 01:45:52 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: Wei Yang <richard.weiyang@...il.com>,
	"David Hildenbrand (Red Hat)" <david@...nel.org>, 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 Fri, Jan 23, 2026 at 11:21:50AM +0800, Qi Zheng wrote:
>
>
>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?
>

Sure.

Since this is trivial cleanup, I will post it till next merge window.

>Thanks,
>Qi
>
>> 
>> > 
>> > -- 
>> > Cheers
>> > 
>> > David
>> 

-- 
Wei Yang
Help you, Help me