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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/4/10 16:14, Barry Song wrote:
> On Wed, Apr 9, 2025 at 1:16 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> 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@linux.alibaba.com>
>> ---
>>   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@kernel.org>

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()?

> 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