Re: [PATCH 5/6] mm/mglru: move folios from oldest gen to second-oldest gen from head to tail

On Fri, Aug 21, 2026 at 6:38 PM Barry Song (Xiaomi) <[email protected]> wrote:
>
> For reclamation, it makes sense to reclaim folios from tail to
> head, as folios near the head are relatively hot. However, when
> moving folios from the oldest generation to the second-oldest
> generation, using the tail-to-head order would effectively cause
> a cold/hot inversion.
>
> Signed-off-by: Barry Song (Xiaomi) <[email protected]>

Hi Barry

This looks a really good idea, thanks!

> ---
>  mm/vmscan.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7bd01875fade..2fd82b2ca4d1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -191,8 +191,20 @@ struct scan_control {
>                         prefetchw(&prev->_field);                       \
>                 }                                                       \
>         } while (0)
> +#define prefetchw_next_lru_folio(_folio, _base, _field)                        \
> +       do {                                                            \
> +               if ((_folio)->lru.next != _base) {                      \
> +                       struct folio *next;                             \
> +                                                                       \
> +                       next = list_entry((_folio)->lru.next,           \
> +                                       struct folio, lru);             \
> +                       prefetchw(&next->_field);                       \
> +               }                                                       \
> +       } while (0)
> +

I got following warning from checkpatch:

    ● checkpatch.pl: 92: WARNING: Argument '_folio' is not used in
function-like macro
    ● checkpatch.pl: 92: WARNING: Argument '_base' is not used in
function-like macro
    ● checkpatch.pl: 92: WARNING: Argument '_field' is not used in
function-like macro

Maybe you could try b4; it helps run these checks automatically. Feel
free to ignore if you think these warning at pointless.

>  #else
>  #define prefetchw_prev_lru_folio(_folio, _base, _field) do { } while (0)
> +#define prefetchw_next_lru_folio(_folio, _base, _field) do { } while (0)
>  #endif
>
>  /*
> @@ -3932,9 +3944,10 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
>         for (zone = 0; zone < MAX_NR_ZONES; zone++) {
>                 struct list_head *head = &lrugen->folios[old_gen][type][zone];
>                 unsigned long protected[MAX_NR_TIERS] = {}, delta = 0;
> +               struct list_head *pos = head->next;
>
> -               while (!list_empty(head)) {
> -                       struct folio *folio = lru_to_folio(head);
> +               while (pos != head) {

Do we need to change the while condition now? Since this commit still
moves folios one by one, will it stop when the list is empty?

> +                       prefetchw_next_lru_folio(folio, head, flags);
> +                       pos = pos->next;

I tried prefetching in MGLRU previously, and it didn't look very good
but there is no regression either; perhaps it's very arch-dependent.
Anyway, I think we can keep it here, maybe further optimize the
prefetch later.

>                         new_gen = __folio_inc_gen(folio, old_gen, &gen_increased);
>                         if (gen_increased) {
>                                 delta += nr_pages;

The rest looks good to me, thanks!