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

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Message-ID: <20260122140034.ymigrfppzwvmcjkr@master>
Date: Thu, 22 Jan 2026 14:00:34 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>
Cc: Wei Yang <richard.weiyang@...il.com>, Qi Zheng <qi.zheng@...ux.dev>,
	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 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.

>
>-- 
>Cheers
>
>David

-- 
Wei Yang
Help you, Help me