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

[Dev Jain <dev.jain@arm.com>]

On 04/11/25 6:43 pm, Lance Yang wrote:
> 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 ;)

Yup, but not sure whether an arm64 enabling patch, only for that to go away
when Qi implements the feature generically, is worth the trouble!

>
>>>>> 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"
>>