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

David Hildenbrand (Red Hat)
2026-01-19T10:20:50Z

On 12/17/25 10:45, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.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.

Right, and if there is ever a need to, I wonder whether that should be a
boottime/runtime toggle instead.

So far we haven't heard of any relevant runtime overheads that causes
problems.

>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.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.

Nothing jumped at me. Hopefully we're not missing something important :)

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

--
Cheers

David