Re: [RFC PATCH] mm: huge_memory: add folio_mark_accessed() when zapping file THP

[Zi Yan]

On 10 Apr 2025, at 6:29, Barry Song wrote:

> On Thu, Apr 10, 2025 at 9:05 PM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 2025/4/10 16:14, Barry Song wrote:
>>> On Wed, Apr 9, 2025 at 1:16 AM Baolin Wang
>>> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>>>>
>>>> When investigating performance issues during file folio unmap, I noticed some
>>>> behavioral differences in handling non-PMD-sized folios and PMD-sized folios.
>>>> For non-PMD-sized file folios, it will call folio_mark_accessed() to mark the
>>>> folio as having seen activity, but this is not done for PMD-sized folios.
>>>>
>>>> This might not cause obvious issues, but a potential problem could be that,
>>>> it might lead to more frequent refaults of PMD-sized file folios under memory
>>>> pressure. Therefore, I am unsure whether the folio_mark_accessed() should be
>>>> added for PMD-sized file folios?
>>>>
>>>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>>>> ---
>>>>   mm/huge_memory.c | 4 ++++
>>>>   1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 6ac6d468af0d..b3ade7ac5bbf 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -2262,6 +2262,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>>>                                  zap_deposited_table(tlb->mm, pmd);
>>>>                          add_mm_counter(tlb->mm, mm_counter_file(folio),
>>>>                                         -HPAGE_PMD_NR);
>>>> +
>>>> +                       if (flush_needed && pmd_young(orig_pmd) &&
>>>> +                           likely(vma_has_recency(vma)))
>>>> +                               folio_mark_accessed(folio);
>>>
>>> Acked-by: Barry Song <baohua@xxxxxxxxxx>
>>
>> Thanks.
>>
>>> I also came across an interesting observation: on a memory-limited system,
>>> demoting unmapped file folios in the LRU—specifically when their mapcount
>>> drops from 1 to 0—can actually improve performance.
>>
>> These file folios are used only once? Can folio_set_dropbehind() be used
>> to optimize it, which can avoid the LRU activity movement in
>> folio_mark_accessed()?
>
> For instance, when a process, such as a game, just exits, it can be expected
> that it won't be used again in the near future. As a result, demoting
> its previously
> unmapped file pages can improve performance.

Is it possible to mark the dying VMAs either VM_SEQ_READ or VM_RAND_READ
so that folio_mark_accessed() will be skipped? Or a new vm_flag?
Will it work?

>
> Of course, for file folios mapped by multiple processes, such as
> common .so files,
> it's a different story. Typically, their mapcounts are always high.

Text VMAs should not be marked.

>
>>
>>> If others have observed the same behavior, we might not need to mark them
>>> as accessed in that scenario.
>>>
>>>>                  }
>>>>
>>>>                  spin_unlock(ptl);
>>>> --
>>>> 2.43.5
>>>>
>>>
>
> Thanks
> Barry


Best Regards,
Yan, Zi