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

[Baolin Wang]

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@xxxxxxxxxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Acked-by: Zi Yan <ziy@xxxxxxxxxx>
>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

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);