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

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Message-ID: <355d3bf3-c6bc-403e-9f19-89259d868611@kernel.org>
Date: Mon, 17 Nov 2025 17:57:58 +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
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-alpha@...r.kernel.org,
 linux-snps-arc@...ts.infradead.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 7/7] mm: make PT_RECLAIM depend on
 MMU_GATHER_RCU_TABLE_FREE && 64BIT

On 14.11.25 12:11, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@...edance.com>

Subject: s/&&/&/

> 
> Make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE so that PT_RECLAIM can
> be enabled by default on all architectures that support
> MMU_GATHER_RCU_TABLE_FREE.
> 
> Considering that a large number of PTE page table pages (such as 100GB+)
> can only be caused on a 64-bit system, let PT_RECLAIM also depend on
> 64BIT.
> 
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
>   arch/x86/Kconfig | 1 -
>   mm/Kconfig       | 6 +-----
>   2 files changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index eac2e86056902..96bff81fd4787 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -330,7 +330,6 @@ 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 a5a90b169435d..e795fbd69e50c 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1440,14 +1440,10 @@ 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
> +	depends on MMU_GATHER_RCU_TABLE_FREE && MMU && SMP && 64BIT

Who would we have MMU_GATHER_RCU_TABLE_FREE without MMU? (can we drop 
the MMU part)

Why do we care about SMP in the first place? (can we frop SMP)

But I also wonder why we need "MMU_GATHER_RCU_TABLE_FREE && 64BIT":

Would it be harmful on 32bit (sure, we might not reclaim as much, but 
still there is memory to be reclaimed?)?

If all 64BIT support MMU_GATHER_RCU_TABLE_FREE (as you previously 
state), why can't we only check for 64BIT?

-- 
Cheers

David