Re: [PATCH v2 2/4] mm/mglru: add PUD-level Bloom filter state and generic helpers

[Kairui Song <ryncsn@gmail.com>]

On Tue, Sep 01, 2026 at 02:37:53PM +0800, Baoquan He wrote:
> Rename the existing PMD-level filter array (filters -> pmd_filters) and
> add a second, coarser pair of PUD-level Bloom filters (pud_filters).
> The PUL filsters operate at 1GB (PUD) granularity, whose 512 PMDs would
> all fail the PMD-level filter - a general suppression of unnecessary page
> table walks. Cross-node empty walks are the most visible case: a foreign
> mm's PUDs have no young entries for this lruvec, so the entire PMD
> iteration is pure waste.
> 
> Generalize the Bloom filter helpers (test/update/reset_bloom_filter ->
> __test/update/reset_bloom_filter) to operate on a generic filters array,
> and add thin wrappers (test/update/reset_pmd_bloom_filter) that pass the
> PMD-level array, so the PUD-level pair can reuse the same hash,
> double-buffering and reset.

Hi Baoquan,

This is really nice idea, thanks! And sorry for the late response.

> 
> No behavior change yet; the PUD filters are populated and used by later
> patches.
> 
> Signed-off-by: Baoquan He <hebaoquan@kylinos.cn>
> ---
>  include/linux/mmzone.h |  6 ++++--
>  mm/vmscan.c            | 45 ++++++++++++++++++++++++++++--------------
>  2 files changed, 34 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f75daa6cd02d..faf28d35065d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -612,8 +612,10 @@ 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 */
> -	unsigned long *filters[NR_BLOOM_FILTERS];
> +	/* PMD-level Bloom filters flip after each iteration */
> +	unsigned long *pmd_filters[NR_BLOOM_FILTERS];
> +	/* PUD-level Bloom filters flip after each iteration */
> +	unsigned long *pud_filters[NR_BLOOM_FILTERS];

Or could we have macros like:
BLOOM_FILTER_LEVELS 2
BLOOM_FILTER_PUD 0
BLOOM_FILTER_PMD 1

(just personal flavor).

>  	/* the mm stats for debugging */
>  	unsigned long stats[NR_HIST_GENS][NR_MM_STATS];
>  };
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 55d43ab54459..cd09c5b8af8c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2842,14 +2842,13 @@ static void get_item_key(void *item, int *key)
>  	key[1] = hash >> BLOOM_FILTER_SHIFT;
>  }
>  
> -static bool test_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long seq,
> -			      void *item)
> +static bool __test_bloom_filter(unsigned long **filters, unsigned long seq, void *item)
>  {
>  	int key[2];
>  	unsigned long *filter;
>  	int gen = filter_gen_from_seq(seq);
>  
> -	filter = READ_ONCE(mm_state->filters[gen]);
> +	filter = READ_ONCE(filters[gen]);
>  	if (!filter)
>  		return true;
>  
> @@ -2858,14 +2857,13 @@ static bool test_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long s
>  	return test_bit(key[0], filter) && test_bit(key[1], filter);
>  }
>  
> -static void update_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long seq,
> -				void *item)
> +static void __update_bloom_filter(unsigned long **filters, unsigned long seq, void *item)
>  {
>  	int key[2];
>  	unsigned long *filter;
>  	int gen = filter_gen_from_seq(seq);
>  
> -	filter = READ_ONCE(mm_state->filters[gen]);
> +	filter = READ_ONCE(filters[gen]);
>  	if (!filter)
>  		return;
>  
> @@ -2877,12 +2875,12 @@ static void update_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long
>  		set_bit(key[1], filter);
>  }
>  
> -static void reset_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long seq)
> +static void __reset_bloom_filter(unsigned long **filters, unsigned long seq)
>  {
>  	unsigned long *filter;
>  	int gen = filter_gen_from_seq(seq);
>  
> -	filter = mm_state->filters[gen];
> +	filter = filters[gen];
>  	if (filter) {
>  		bitmap_clear(filter, 0, BIT(BLOOM_FILTER_SHIFT));
>  		return;
> @@ -2890,7 +2888,24 @@ static void reset_bloom_filter(struct lru_gen_mm_state *mm_state, unsigned long
>  
>  	filter = bitmap_zalloc(BIT(BLOOM_FILTER_SHIFT),
>  			       __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN);
> -	WRITE_ONCE(mm_state->filters[gen], filter);
> +	WRITE_ONCE(filters[gen], filter);

So the helpers will be something like
test_bloom_filter(mm_state, level, ...), seems a bit cleaner?