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

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

On 2025/4/9 17:46, David Hildenbrand wrote:
> On 09.04.25 11:38, Baolin Wang 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 reclaim hot file folios under memory pressure, as quoted
>> from Johannes:
>>
>> "
>> Sometimes file contents are only accessed through relatively short-lived
>> mappings. But they can nevertheless be accessed a lot and be hot. It's
>> important to not lose that information on unmap, and end up kicking out a
>> frequently used cache page.
>> "
>>
>> Therefore, we should also add folio_mark_accessed() for PMD-sized file
>> folios when unmapping.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> ---
>> Changes from RFC:
>>   - Update the commit message, per Johannes.
>>   - Collect Acked tags from Johannes and Zi. Thanks.
>> ---
>>   mm/huge_memory.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 2a47682d1ab7..955781b4e946 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2260,6 +2260,10 @@ 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);
> 
> So the flush_needed check is really just a pmd_present() check. (the 
> latter would be clearer, but I don't mind)

Yes, we've already checked pmd_present() before, so I assume the 
flush_needed check is cheaper:)

> Acked-by: David Hildenbrand <david@redhat.com>

Thanks.