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

[Qi Zheng <zhengqi.arch@bytedance.com>]

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.

> 
> 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@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"
>>