Re: [PATCH RFC v3 13/17] mm/mglru: make folio_inc_lru_refs lruvec lockless

[KunWu Chan]

On Sat, Oct 3, 2026 at 8:55 PM Kairui Song via B4 Relay
<devnull+kasong.tencent.com@xxxxxxxxxx> wrote:
>
> From: Kairui Song <kasong@xxxxxxxxxxx>
>
> The lruvec spinlock in folio_inc_lru_refs was only taken to keep
> concurrent aging from corrupting the size counters.  This is no
> longer needed: the per-generation counters are atomic, and the
> active/inactive ABI counters are updated per folio and linearized by
> the CAS on the folio flags, so concurrent aging can no longer corrupt
> them.  The same argument already allowed folio_reset_lru_refs() to
> drop the lock.
>
> Use folio_lruvec_live_get()/folio_lruvec_live_put() to hold the RCU
> read lock around the lruvec lookup, and drop the spinlock entirely.
>
> Without the lock, aging can advance max_seq while a promotion is in
> flight.  Generations are indexed by seq % MAX_NR_GENS, so the folio
> flags can return to the value read earlier and the cmpxchg succeeds
> with a target gen computed from a stale max_seq.  Recheck max_seq after
> a gen-changing cmpxchg: if it moved, account the committed update and
> retry with LRU_REF_FORCE, which promotes the folio to the newest gen.
> The retry may count the access twice, which is acceptable and avoids
> folio_activate(), whose LRU lock causes latency jitter.  smp_rmb()
> orders the flags read, including the one returned by a failed
> cmpxchg(), before the max_seq read.
>
> Also simplify the post-CAS accounting guards: gen is only assigned
> non-negative values after the old_gen < 0 early exit, so gen != old_gen
> implies gen >= 0, and lru_gen_update_size() needs no explicit guard.
> The active/inactive ABI update keeps its gen >= 0 check because an
> off-LRU folio has no lruvec to update.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
> ---
>  include/linux/mm_inline.h | 10 +++++-----
>  include/linux/mmzone.h    |  3 ++-
>  mm/vmscan.c               | 47 ++++++++++++++++++++++++++++-------------------
>  3 files changed, 35 insertions(+), 25 deletions(-)
>
> diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
> index ee7fcbe2366b..b68d68101248 100644
> --- a/include/linux/mm_inline.h
> +++ b/include/linux/mm_inline.h
> @@ -293,10 +293,9 @@ static inline int folio_lru_gen(const struct folio *folio)
>         return lru_get_gen_flags(READ_ONCE(*const_folio_flags(folio, 0)));
>  }
>
> -static inline void lru_gen_update_size(struct lruvec *lruvec, struct folio *folio,
> -                                      int old_gen, int new_gen)
> +static inline void lru_gen_update_size(struct lruvec *lruvec, int type,
> +               struct folio *folio, int old_gen, int new_gen)
>  {
> -       int type = folio_is_file_lru(folio);
>         int zone = folio_zonenum(folio);
>         int delta = folio_nr_pages(folio);
>         struct lru_gen_folio *lrugen = &lruvec->lrugen;
> @@ -371,7 +370,7 @@ static inline bool lru_gen_add_folio(struct lruvec *lruvec, struct folio *folio,
>         /* use the refs from the atomic snapshot to avoid raced update */
>         refs = lru_get_refs_flags(flags);
>
> -       lru_gen_update_size(lruvec, folio, -1, gen);
> +       lru_gen_update_size(lruvec, type, folio, -1, gen);
>         if (lru_refs_is_active(refs))
>                 lru += LRU_ACTIVE;
>         __update_lru_size(lruvec, lru, zone, delta);
> @@ -389,6 +388,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>  {
>         unsigned long flags;
>         int gen, refs;
> +       int type = folio_is_file_lru(folio);
>         int zone = folio_zonenum(folio);
>         int delta = folio_nr_pages(folio);
>         enum lru_list lru = folio_is_file_lru(folio) * LRU_INACTIVE_FILE;
> @@ -411,7 +411,7 @@ static inline bool lru_gen_del_folio(struct lruvec *lruvec, struct folio *folio,
>         if (!reclaiming && ((max_seq - gen) % MAX_NR_GENS) < MIN_NR_GENS)
>                 folio_set_active(folio);
>
> -       lru_gen_update_size(lruvec, folio, gen, -1);
> +       lru_gen_update_size(lruvec, type, folio, gen, -1);
>         if (lru_refs_is_active(refs))
>                 lru += LRU_ACTIVE;
>         __update_lru_size(lruvec, lru, zone, -delta);
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e096fd0a5047..d78e3f97423e 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -558,9 +558,10 @@ enum lruvec_flags {
>  #define LRU_TIER_MIN           0U
>  #define LRU_TIER_MAX           (MAX_NR_TIERS - 1)
>
> -/* Access source flags for folio_inc_lru_refs() */
> +/* Flags for folio_inc_lru_refs() */
>  #define LRU_REF_MAPPED         0x1U
>  #define LRU_REF_EXEC           0x2U
> +#define LRU_REF_FORCE          0x4U
>
>  #define LRU_REFS_REFERENCED    0x1
>  #define LRU_REFS_WORKINGSET    0x2
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ae4a0522a8d5..9cc06a95d931 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -982,38 +982,33 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
>         long nr_pages = folio_nr_pages(folio);
>         struct lru_gen_folio *lrugen;
>         struct lruvec *lruvec = NULL;
> +       bool aged = false;
>
> +retry:
>         old_flags = READ_ONCE(*folio_flags(folio, 0));
>         do {
>                 new_flags = old_flags;
>                 old_gen = lru_get_gen_flags(old_flags);
>                 old_refs = lru_get_refs_flags(old_flags);
> -               file = folio_flags_is_file_lru(&old_flags);
>                 refs = old_refs + 1;
>                 gen = old_gen;
> +               file = folio_flags_is_file_lru(&old_flags);
>                 if (old_gen < 0)
>                         goto out;
> -               /*
> -                * Lock the lruvec if the folio is on-list. We are already
> -                * doing lazy promotion so in theory we don't need this,
> -                * but for now, concurrent aging would still corrupt the
> -                * size counters.  This is a temporary limitation and
> -                * will be lifted very soon, so the lock here is not a
> -                * performance concern.
> -                */
>                 if (!lruvec) {
> -                       lruvec = lruvec_live_lock_irq(folio_lruvec(folio));
> +                       lruvec = folio_lruvec_live_get(folio);
>                         lrugen = &lruvec->lrugen;
>                 }
> +               /* Failed cmpxchg() does not order old_flags before max_seq. */
> +               smp_rmb();
>                 max_seq = READ_ONCE(lrugen->max_seq);
>                 max_gen = lru_gen_from_seq(max_seq);
>                 min_gen = lru_gen_from_seq(READ_ONCE(lrugen->min_seq[file]));
>                 if (old_gen == max_gen)
>                         goto out;
> -
> -               if (flags & (LRU_REF_MAPPED | LRU_REF_EXEC)) {
> -                       /* Promote second page table access or executable */
> -                       if (refs > LRU_REFS_REFERENCED || flags & LRU_REF_EXEC)
> +               if (flags & (LRU_REF_MAPPED | LRU_REF_EXEC | LRU_REF_FORCE)) {
> +                       if (refs > LRU_REFS_REFERENCED ||
> +                           flags & (LRU_REF_EXEC | LRU_REF_FORCE))
>                                 gen = max_gen;
>                         /* First access only defers eviction from the oldest gen */
>                         else if (old_gen == min_gen)
> @@ -1037,9 +1032,19 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
>                         break;
>         } while (!try_cmpxchg(folio_flags(folio, 0), &old_flags, new_flags));
>
> -       if (gen != old_gen)
> -               lru_gen_update_size(lruvec, folio, old_gen, gen);
> -       if (lru_refs_is_active(old_refs) != lru_refs_is_active(refs) && old_gen >= 0) {
> +       if (gen != old_gen) {
> +               /*
> +                * Gen-index reuse can fool cmpxchg(). Retry locklessly, accepting
> +                * an extra reference to avoid folio_activate() latency.
> +                */
> +               if (unlikely(READ_ONCE(lrugen->max_seq) != max_seq)) {
> +                       flags = LRU_REF_FORCE;
> +                       aged = true;
> +               }
> +               lru_gen_update_size(lruvec, file, folio, old_gen, gen);
> +       }
> +
> +       if (lru_refs_is_active(old_refs) != lru_refs_is_active(refs) && gen >= 0) {
>                 enum lru_list lru = file * LRU_INACTIVE_FILE;
>

Hi Kairui,

One question about the retry path.

When `max_seq` changes after a gen-changing cmpxchg, the retry uses
`LRU_REF_FORCE` and promotes the folio directly to `max_gen`. In the
normal first `LRU_REF_MAPPED` access path, the same folio is only moved
to the next generation when it is in the oldest generation.

Is the stronger promotion on the retry intentional? In other words,
does detecting that `max_seq` advanced during the update justify
promoting the folio all the way to the newest generation, rather than
re-evaluating the original promotion policy with the new `max_seq`?

Thanks,
Kunwu


>                 __update_lru_size(lruvec, lru + lru_refs_is_active(old_refs),
> @@ -1047,8 +1052,12 @@ void folio_inc_lru_refs(struct folio *folio, unsigned int flags)
>                 __update_lru_size(lruvec, lru + lru_refs_is_active(refs),
>                                   folio_zonenum(folio), nr_pages);
>         }
> +       if (aged) {
> +               aged = false;
> +               goto retry;
> +       }
>         if (lruvec)
> -               lruvec_unlock_irq(lruvec);
> +               folio_lruvec_live_put(lruvec);
>  }
>
>  /*
> @@ -3648,7 +3657,7 @@ static int folio_inc_gen(struct lruvec *lruvec, struct folio *folio)
>
>         new_gen = __folio_inc_gen(lruvec, folio, old_gen, &gen_increased);
>         if (gen_increased)
> -               lru_gen_update_size(lruvec, folio, old_gen, new_gen);
> +               lru_gen_update_size(lruvec, type, folio, old_gen, new_gen);
>
>         return new_gen;
>  }
>
> --
> 2.55.0
>
>