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

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

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

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.


>
> 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"
>