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

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

On 2025/4/10 16:45, Oscar Salvador wrote:
> On Wed, Apr 09, 2025 at 05:38:58PM +0800, 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>
> 
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Thanks.

> Although I agree with David here that pmd_present would be more obvious than
> flush_needed.
> It was not obvious to be at first glance.

How about adding some comments to make it clear?

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b3ade7ac5bbf..93abd1fcc4fb 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2263,6 +2263,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct 
vm_area_struct *vma,
                         add_mm_counter(tlb->mm, mm_counter_file(folio),
                                        -HPAGE_PMD_NR);

+                       /*
+                        * Use flush_needed to indicate whether the PMD 
entry is present,
+                        * instead of checking pmd_present() again.
+                        */
                         if (flush_needed && pmd_young(orig_pmd) &&
                             likely(vma_has_recency(vma)))
                                 folio_mark_accessed(folio);