Re: [RFC PATCH 1/6] mm/mglru: add PUD-level Bloom filter state

On Thu, Aug 6, 2026 at 6:30 PM Baoquan He <[email protected]> wrote:
>
> Add a second, coarser pair of double-buffered Bloom filters to
> struct lru_gen_mm_state. Like the PMD-level filters, they flip
> each generation, but they operate at 1GB (PUD) granularity so the
> page table walker can skip whole PUD subtrees whose 512 PMDs would
> all fail the PMD-level filter. This mainly targets the page tables
> of processes whose memory lives only on other NUMA nodes
> (cross-node empty walks).
>
> No behavior change yet; the filters are populated and consumed by
> follow-up patches.
>
> Signed-off-by: Baoquan He <[email protected]>
> ---
>  include/linux/mmzone.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index a26c8b855222..2bda24522d9f 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -608,8 +608,16 @@ struct lru_gen_mm_state {
>         struct list_head *head;
>         /* where the last iteration ended before */
>         struct list_head *tail;
> -       /* Bloom filters flip after each iteration */
> +       /* PMD-level Bloom filters flip after each iteration */
>         unsigned long *filters[NR_BLOOM_FILTERS];
> +       /*
> +        * PUD-level Bloom filters flip after each iteration. Same double
> +        * buffering as the PMD-level filters, but coarser: they remember
> +        * which 1GB PUD subtrees had young leaf entries last generation,
> +        * so walk_pud_range() can skip whole subtrees whose PMD iteration
> +        * would find nothing worth scanning (e.g. cross-node empty walks).
> +        */
> +       unsigned long *pud_filters[NR_BLOOM_FILTERS];

Could we rename them to pmd_filters and pud_filters rather
than filters and pud_filters? The current naming looks a bit
inconsistent.

I also feel the names are quite self-explanatory, so perhaps we don't
need such a long comment here?

>         /* the mm stats for debugging */
>         unsigned long stats[NR_HIST_GENS][NR_MM_STATS];
>  };
> --
> 2.54.0
>

Best Regards
Barry