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

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Message-ID: <8e164e8b-1748-4570-822c-b973770e8fb7@kernel.org>
Date: Mon, 19 Jan 2026 11:20:44 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: 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
Cc: 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 12/17/25 10:45, 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.

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@...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.


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

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

-- 
Cheers

David