Re: [PATCH v4 6/6] mm: madvise: Avoid split during MADV_PAGEOUT and MADV_COLD

[Ryan Roberts]

>>>>>>>> -               VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
>>>>>>>> -
>>>>>>>> -               if (!pageout && pte_young(ptent)) {
>>>>>>>> -                       ptent = ptep_get_and_clear_full(mm, addr, pte,
>>>>>>>> -                                                       tlb->fullmm);
>>>>>>>> -                       ptent = pte_mkold(ptent);
>>>>>>>> -                       set_pte_at(mm, addr, pte, ptent);
>>>>>>>> -                       tlb_remove_tlb_entry(tlb, pte, addr);
>>>>>>>> +               if (!pageout) {
>>>>>>>> +                       for (; nr != 0; nr--, pte++, addr += PAGE_SIZE) {
>>>>>>>> +                               if (ptep_test_and_clear_young(vma, addr, pte))
>>>>>>>> +                                       tlb_remove_tlb_entry(tlb, pte, addr);
>>>>>
>>>>> IIRC, some of the architecture(ex, PPC) don't update TLB with set_pte_at and
>>>>> tlb_remove_tlb_entry. So, didn't we consider remapping the PTE with old after
>>>>> pte clearing?
>>>>
>>>> Sorry Lance, I don't understand this question, can you rephrase? Are you saying
>>>> there is a good reason to do the original clear-mkold-set for some arches?
>>>
>>> IIRC, some of the architecture(ex, PPC)  don't update TLB with
>>> ptep_test_and_clear_young()
>>> and tlb_remove_tlb_entry().

Afraid I'm still struggling with this comment. Do you mean to say that powerpc
invalidates the TLB entry as part of the call to ptep_test_and_clear_young()? So
tlb_remove_tlb_entry() would be redundant here, and likely cause performance
degradation on that architecture?

IMHO, ptep_test_and_clear_young() really shouldn't be invalidating the TLB
entry, that's what ptep_clear_flush_young() is for.

But I do see that for some cases of the 32-bit ppc, there appears to be a flush:

#define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
static inline int __ptep_test_and_clear_young(struct mm_struct *mm,
					      unsigned long addr, pte_t *ptep)
{
	unsigned long old;
	old = pte_update(mm, addr, ptep, _PAGE_ACCESSED, 0, 0);
	if (old & _PAGE_HASHPTE)
		flush_hash_entry(mm, ptep, addr);   <<<<<<<<

	return (old & _PAGE_ACCESSED) != 0;
}
#define ptep_test_and_clear_young(__vma, __addr, __ptep) \
	__ptep_test_and_clear_young((__vma)->vm_mm, __addr, __ptep)

Is that what you are describing? Does any anyone know why flush_hash_entry() is
called? I'd say that's a bug in ppc and not a reason not to use
ptep_test_and_clear_young() in the common code!

Thanks,
Ryan


>>
>> Err, I assumed tlb_remove_tlb_entry() meant "invalidate the TLB entry for this
>> address please" - albeit its deferred and batched. I'll look into this.
>>
>>>
>>> In my new patch[1], I use refresh_full_ptes() and
>>> tlb_remove_tlb_entries() to batch-update the
>>> access and dirty bits.
>>
>> I want to avoid the per-pte clear-modify-set approach, because this doesn't
>> perform well on arm64 when using contpte mappings; it will cause the contpe
>> mapping to be unfolded by the first clear that touches the contpte block, then
>> refolded by the last set to touch the block. That's expensive.
>> ptep_test_and_clear_young() doesn't suffer that problem.
> 
> Thanks for explaining. I got it.
> 
> I think that other architectures will benefit from the per-pte clear-modify-set
> approach. IMO, refresh_full_ptes() can be overridden by arm64.
> 
> Thanks,
> Lance
>>
>>>
>>> [1] https://lore.kernel.org/linux-mm/20240316102952.39233-1-ioworker0@xxxxxxxxx
>>>
>>> Thanks,
>>> Lance
>>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Lance
>>>>>
>>>>>
>>>>>
>>>>>>>> +                       }
>>>>>>>
>>>>>>> This looks so smart. if it is not pageout, we have increased pte
>>>>>>> and addr here; so nr is 0 and we don't need to increase again in
>>>>>>> for (; addr < end; pte += nr, addr += nr * PAGE_SIZE)
>>>>>>>
>>>>>>> otherwise, nr won't be 0. so we will increase addr and
>>>>>>> pte by nr.
>>>>>>
>>>>>> Indeed. I'm hoping that Lance is able to follow a similar pattern for
>>>>>> madvise_free_pte_range().
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>                 }
>>>>>>>>
>>>>>>>>                 /*
>>>>>>>> --
>>>>>>>> 2.25.1
>>>>>>>>
>>>>>>>
>>>>>>> Overall, LGTM,
>>>>>>>
>>>>>>> Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>>
>>>>
>>