Re: [RFC PATCH] mm: Enable CONFIG_PT_RECLAIM on all architectures

[Qi Zheng]

Hi Dev,

On 11/3/25 2:37 PM, Dev Jain wrote:
> The implementation of CONFIG_PT_RECLAIM is completely contained in generic
> mm code. It depends on the RCU callback which will reclaim the pagetables -
> there is nothing arch-specific about that. So, enable this config for
> all architectures.

Thanks for doing this!

But unfortunately, not all architectures call tlb_remove_ptdesc() in
__pte_free_tlb(). Some architectures directly call pte_free() to
free PTE pages (without RCU).

We need to modify these architectures first, otherwise it will
lead to UAF. This approach is feasible because Hugh provides similar
support in pte_free_defer().

Enabling PT_RECLAIM on all architecture has always been on my
TODO list, but it's been blocked by other things. :(

Thanks,
Qi

> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> ---
>   arch/x86/Kconfig | 1 -
>   mm/Kconfig       | 5 +----
>   mm/pt_reclaim.c  | 2 +-
>   3 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index fa3b616af03a..5681308a5650 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -327,7 +327,6 @@ 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 0e26f4fc8717..903c37d02555 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1355,13 +1355,10 @@ 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
> +	depends on MMU && SMP
>   	select MMU_GATHER_RCU_TABLE_FREE
>   	help
>   	  Try to reclaim empty user page table pages in paths other than munmap
> diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
> index 7e9455a18aae..049e17f08c6a 100644
> --- a/mm/pt_reclaim.c
> +++ b/mm/pt_reclaim.c
> @@ -1,6 +1,6 @@
>   // SPDX-License-Identifier: GPL-2.0
>   #include <linux/hugetlb.h>
> -#include <asm-generic/tlb.h>
> +#include <asm/tlb.h>
>   #include <asm/pgalloc.h>
>   
>   #include "internal.h"