linux-kernel - Re: [PATCH 7/7] mm: make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE

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Message-ID: <195baf7c-1f4e-46a4-a4aa-e68e7d00c0f9@linux.dev>
Date: Tue, 18 Nov 2025 20:02:30 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: "David Hildenbrand (Red Hat)" <david@...nel.org>, will@...nel.org,
 aneesh.kumar@...nel.org, npiggin@...il.com, peterz@...radead.org,
 dev.jain@....com, akpm@...ux-foundation.org, ioworker0@...il.com
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-alpha@...r.kernel.org,
 linux-snps-arc@...ts.infradead.org, loongarch@...ts.linux.dev,
 linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
 linux-um@...ts.infradead.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH 7/7] mm: make PT_RECLAIM depend on
 MMU_GATHER_RCU_TABLE_FREE && 64BIT



On 11/18/25 12:57 AM, David Hildenbrand (Red Hat) wrote:
> On 14.11.25 12:11, Qi Zheng wrote:
>> From: Qi Zheng <zhengqi.arch@...edance.com>
> 
> Subject: s/&&/&/

will do.

> 
>>
>> Make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE so that PT_RECLAIM 
>> can
>> be enabled by default on all architectures that support
>> MMU_GATHER_RCU_TABLE_FREE.
>>
>> Considering that a large number of PTE page table pages (such as 100GB+)
>> can only be caused on a 64-bit system, let PT_RECLAIM also depend on
>> 64BIT.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>>   arch/x86/Kconfig | 1 -
>>   mm/Kconfig       | 6 +-----
>>   2 files changed, 1 insertion(+), 6 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index eac2e86056902..96bff81fd4787 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -330,7 +330,6 @@ 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 a5a90b169435d..e795fbd69e50c 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -1440,14 +1440,10 @@ 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
>> -    select MMU_GATHER_RCU_TABLE_FREE
>> +    depends on MMU_GATHER_RCU_TABLE_FREE && MMU && SMP && 64BIT
> 
> Who would we have MMU_GATHER_RCU_TABLE_FREE without MMU? (can we drop 
> the MMU part)

OK.

> 
> Why do we care about SMP in the first place? (can we frop SMP)

OK.

> 
> But I also wonder why we need "MMU_GATHER_RCU_TABLE_FREE && 64BIT":
> 
> Would it be harmful on 32bit (sure, we might not reclaim as much, but 
> still there is memory to be reclaimed?)?

This is also fine on 32bit, but the benefits are not significant, So I
chose to enable it only on 64-bit.

I actually tried enabling MMU_GATHER_RCU_TABLE_FREE on all
architectures, and apart from sparc32 being a bit troublesome (because
it uses mm->page_table_lock for synchronization within
__pte_free_tlb()), the modifications were relatively simple.

> 
> If all 64BIT support MMU_GATHER_RCU_TABLE_FREE (as you previously 
> state), why can't we only check for 64BIT?

OK, will do.

Thanks,
Qi

>