Re: [PATCH v2] mm/mglru: fix a no-op lru_gen_clear_refs() call in lru_lazyfree()

[Barry Song]

On Tue, Oct 27, 2026 at 7:40 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Mon, Oct 26, 2026 at 4:41 PM Hui Zhu <hui.zhu@xxxxxxxxx> wrote:
> >
> > From: Hui Zhu <zhuhui@xxxxxxxxxx>
> >
> > lru_lazyfree() calls lru_gen_clear_refs() after lruvec_del_folio(), but
> > lru_gen_del_folio() has already cleared LRU_GEN_MASK, so folio_lru_gen()
> > returns -1 and lru_gen_clear_refs() exits at its "gen < 0" check without
> > clearing the refs or PG_workingset.
> >
> > A MADV_FREE folio therefore keeps the state it had before madvise(2),
> > even though madvise_free_pte_range() has already cleared the accessed and
> > dirty bits of its PTEs.  lru_gen_folio_seq() places it one generation
> > younger, and PG_workingset puts it in the last tier, so sort_folio()
> > promotes and protects it instead of evicting it and charges it to
> > lrugen->protected[], which read_ctrl_pos() uses to balance anon against
> > file reclaim.  __folio_inc_gen() clears the refs but not PG_workingset,
> > so this repeats every time the folio reaches the oldest generation.  If
> > the folio is redirtied first, ttu_anon_lazyfree_folio() keeps it and
> > lru_gen_eviction() packs the stale state into the shadow entry, so a
> > later refault is promoted to the youngest generation.
> >
> > Call lru_gen_clear_refs() while the folio is still on the generation
> > list, as deactivate_file_folio() and folio_deactivate() already do.  Its
> > return value only says whether the folio is in the oldest generation,
> > which lru_lazyfree() does not need because it re-adds the folio below.
> > Keep folio_clear_referenced() unconditional: the classic LRU needs it,
> > and it also covers the folios that lru_gen_clear_refs() skips.
> >
> > Fixes: cc8ec7be78ff ("mm/mglru: optimize deactivation")
> > Suggested-by: Barry Song <baohua@xxxxxxxxxx>
> > Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> > ---
> > Changelog:
> > v2:
> > Call lru_gen_clear_refs() before lruvec_del_folio() instead of
> > open-coding the flag clears; drop the lru_gen_enabled() branch and
> > the comment (Barry Song).
> >
> >  mm/folio.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/folio.c b/mm/folio.c
> > index 35e242b48870..ea3adb6a89d7 100644
> > --- a/mm/folio.c
> > +++ b/mm/folio.c
> > @@ -597,12 +597,10 @@ static void lru_lazyfree(struct lruvec *lruvec, struct folio *folio)
> >             folio_test_swapcache(folio) || folio_test_unevictable(folio))
> >                 return;
> >
> > +       lru_gen_clear_refs(folio);
> >         lruvec_del_folio(lruvec, folio);
> >         folio_clear_active(folio);
> > -       if (lru_gen_enabled())
> > -               lru_gen_clear_refs(folio);
> > -       else
> > -               folio_clear_referenced(folio);
> > +       folio_clear_referenced(folio);
>
> Why can't we just call folio_set_lru_refs(.., 0), and should we clear
> PG_workingset? Keeping it helps PSI on refualt / redirty tracking (not
> sure if meaningful but CLRU does this), clearing it makes eviction
> better, but the eviction issue is MGLRU-only, CLRU keeps it.

I recall that in v1 [1], Hui mentioned:

"Use folio_clear_referenced() rather than
folio_set_lru_refs(), which is not defined without
CONFIG_LRU_GEN."

I assume we can support this, but doesn't the code below
also look a bit weird?


diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index 829977d399e6..66ed805a48f5 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -431,6 +431,15 @@ static inline void folio_migrate_lru_refs(struct
folio *new, const struct folio
        if (folio_test_referenced(old))
                folio_set_referenced(new);
 }
+
+static inline void folio_set_lru_refs(struct folio *folio, unsigned int refs)
+{
+       if (!refs)
+               folio_clear_referenced(folio);
+       else if (refs == 1)
+               folio_set_referenced(folio);
+}
+
 #endif /* CONFIG_LRU_GEN */

[1] https://lore.kernel.org/linux-mm/20261023080430.465305-1-hui.zhu@xxxxxxxxx/

Best,
Barry