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

["David Hildenbrand (Red Hat)" <david@kernel.org>]

On 19.11.25 12:02, Qi Zheng wrote:
> 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.
> 

I guess if we just make it depend on MMU_GATHER_RCU_TABLE_FREE that will 
be fine.

> [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.

I was briefly wondering if on 32bit (but maybe also on 64bit with 
configurable user page table levels?) we could have the scenario that we 
only have two page table levels.

So reclaiming the PMD level (corresponding to the highest level) would 
be impossible. But for that to happen one would have to discard the 
whole address range through MADV_DONTNEED (impossible I guess) :)

-- 
Cheers

David