linux-kernel - Re: [RFC PATCH] mm: Enable CONFIG_PT

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Message-ID: <2be04785-d725-4e79-a609-87f174271f83@arm.com>
Date: Tue, 4 Nov 2025 09:32:25 +0530
From: Dev Jain <dev.jain@....com>
To: Qi Zheng <zhengqi.arch@...edance.com>, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
 catalin.marinas@....com, will@...nel.org, akpm@...ux-foundation.org,
 david@...hat.com, hannes@...xchg.org
Cc: ryan.roberts@....com, hpa@...or.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, vbabka@...e.cz, ppt@...nel.org, surenb@...gle.com,
 mhocko@...e.com, shakeel.butt@...ux.dev, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [RFC PATCH] mm: Enable CONFIG_PT_RECLAIM on all architectures


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!


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