Re: [PATCH v3 7/7] mm: make PT_RECLAIM depends on MMU_GATHER_RCU_TABLE_FREE

[Qi Zheng]

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@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
> > ---
> > 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.