Re: [PATCH 7/7] mm: make PT_RECLAIM depend on MMU_GATHER_RCU_TABLE_FREE && 64BIT

[Qi Zheng <qi.zheng@linux.dev>]

Hi David,

On 11/19/25 6:19 PM, David Hildenbrand (Red Hat) wrote:
> On 18.11.25 13:02, Qi Zheng wrote:
>>
>>
>> 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@bytedance.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@bytedance.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.
> 
> Right. Address space is smaller, but also memory is smaller. Not that I 
> think we strictly *must* to support 32bit, I merely wonder why we 
> wouldn't just enable it here.
> 
> OTOH, if there is a good reason we cannot enable it, we can definitely 
> just keep it 64bit only.

The only difficulty is this:

> 
>>
>> 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.

in sparc32:

void pte_free(struct mm_struct *mm, pgtable_t ptep)
{
         struct page *page;

         page = pfn_to_page(__nocache_pa((unsigned long)ptep) >> 
PAGE_SHIFT);
         spin_lock(&mm->page_table_lock);
         if (page_ref_dec_return(page) == 1)
                 pagetable_dtor(page_ptdesc(page));
         spin_unlock(&mm->page_table_lock);

         srmmu_free_nocache(ptep, SRMMU_PTE_TABLE_SIZE);
}

#define __pte_free_tlb(tlb, pte, addr)  pte_free((tlb)->mm, pte)

To enable MMU_GATHER_RCU_TABLE_FREE on sparc32, we need to implement
__tlb_remove_table(), and call the pte_free() above in __tlb_remove_table().

However, the __tlb_remove_table() does not have an mm parameter:

void __tlb_remove_table(void *_table)

so we need to use another lock instead of mm->page_table_lock.

I have already sent the v2 [1], and perhaps after that I can enable
PT_RECLAIM on all 32-bit architectures as well.

[1]. 
https://lore.kernel.org/all/cover.1763537007.git.zhengqi.arch@bytedance.com/

>>
>>>
>>> If all 64BIT support MMU_GATHER_RCU_TABLE_FREE (as you previously
>>> state), why can't we only check for 64BIT?
>>
>> OK, will do.
> 
> This was also more of a question for discussion:
> 
> Would it make sense to have
> 
> config PT_RECLAIM
>      def_bool y
>      depends on MMU_GATHER_RCU_TABLE_FREE

make sense.

> 
> (a) Would we want to make it configurable (why?)

No, it was just out of caution before.

> (b) Do we really care about SMP (why?)

No. Simply because the following situation is impossible to occur:

pte_offset_map
traversing the PTE page table

<preemption or hardirq>

call madvise(MADV_DONTNEED)

so there's no need to free PTE page via RCU.

> (c) Do we want to limit to 64bit (why?)

No, just because the profit is greater at 64-BIT.

> (d) Do we really need the MMU check in addition to
>      MMU_GATHER_RCU_TABLE_FREE

No, I was worried about compilation issues before, but now it seems that
my worries were unnecessary.

Thanks,
Qi

> 
>