Re: [PATCH v6 1/5] mm: rmap: support batched checks of the references for large folios

[David Hildenbrand (Arm)]

On 3/16/26 07:25, Baolin Wang wrote:
> 
> 
> On 3/10/26 4:17 PM, David Hildenbrand (Arm) wrote:
>> On 3/10/26 02:37, Baolin Wang wrote:
>>>
>>>
>>>
>>> I understand your point. I’m concerned that I can’t test this patch on
>>> every architecture to validate the benefits. Anyway, let me try this on
>>> my X86 machine first.
>>
>> In any case, please make that a follow-up patch :)
> 
> Sure. However, after investigating RISC‑V and x86, I found that
> ptep_clear_flush_young() does not flush the TLB on these architectures:
> 
> int ptep_clear_flush_young(struct vm_area_struct *vma,
>                unsigned long address, pte_t *ptep)
> {
>     /*
>      * On x86 CPUs, clearing the accessed bit without a TLB flush
>      * doesn't cause data corruption. [ It could cause incorrect
>      * page aging and the (mistaken) reclaim of hot pages, but the
>      * chance of that should be relatively low. ]
>      *
>      * So as a performance optimization don't flush the TLB when
>      * clearing the accessed bit, it will eventually be flushed by
>      * a context switch or a VM operation anyway. [ In the rare
>      * event of it not getting flushed for a long time the delay
>      * shouldn't really matter because there's no real memory
>      * pressure for swapout to react to. ]
>      */
>     return ptep_test_and_clear_young(vma, address, ptep);
> }

You'd probably want an arch helper then, that tells you whether
a flush_tlb_range() after ptep_test_and_clear_young() is required.

Or some special flush_tlb_range() helper.

I agree that it requires more work.

-- 
Cheers,

David