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

[Lance Yang <ioworker0@gmail.com>]

From: Lance Yang <lance.yang@linux.dev>


On Tue, 4 Nov 2025 14:33:00 +0800, Qi Zheng wrote:
> 
> 
> On 11/4/25 12:02 PM, Dev Jain wrote:
> > 
> > On 03/11/25 2:37 pm, Qi Zheng wrote:
> >> Hi Dev,
> >>
> >> On 11/3/25 4:43 PM, Dev Jain wrote:
> >>>
> >>> On 03/11/25 12:33 pm, Qi Zheng wrote:
> >>>> 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).
> >>>
> >>> Thanks! This was not obvious to figure out.
> >>>
> >>> Is there an arch bottleneck because of which they do this? I mean to 
> >>> say,
> >>>
> >>> is something stopping us from simply redirecting __pte_free_tlb to 
> >>> tlb_remove_ptdesc
> >>
> >> Some architectures have special handling in __pte_free_tlb(), and cannot
> >> simple redirect __pte_free_tlb() to tlb_remove_ptdesc(), such as m68k,
> >> powerpc, etc.
> >>
> >> For those architectures that call pte_free() in __pte_free_tlb(), it
> >> should be easy to modify them.
> >>
> >> If you're not in a rush, I can take the time to finish the above tasks.
> > 
> > Right then, I'll leave that up to you!
> 
> OK, I will do it ASAP.

Cool! Looking forward to seeing that land ;p

Cheers,
Lance

> 
> > 
> > 
> >>
> >>>
> >>> or pte_free_defer?
> >>>
> >>>
> >>> I am looking to enable this config at least on arm64 by default, I 
> >>> believe it will be legal

Great proposal, Dev! That looks like a very useful feature. Let's make it
happen on arm64 ;)

> >>>
> >>> to do this at least here.
> >>
> >> IIRC, arm64 can directly enable CONFIG_PT_RECLAIM, as it is supported
> >> at the architecture level.
> >>
> >> Thanks,
> >> Qi
> >>
> >>>
> >>>
> >>>>
> >>>> 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@arm.com>
> >>>>> ---
> >>>>>   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"
> >>>>
> >>
> 
>