Re: [PATCH v2 3/4] mm/mglru: skip cold PUD subtrees during aging

[Kairui Song <ryncsn@gmail.com>]

On Tue, Sep 01, 2026 at 02:37:54PM +0800, Baoquan He wrote:
> The aging walks into every PUD and runs the PMD-level Bloom filter
> on each PMD. Add a coarser PUD-level filter (pud_filters) one level
> up:
>  - walk_pmd_range() now reports whether it found any young leaf entries,
>  - walk_pud_range() records that in the PUD filter, and
>  - on subsequent generations, skips the whole 1GB subtree when the filter
>    says it had none last generation.
> 
> The double-buffered filter flips with each new iteration, so newly hot or
> migrated-in pages are re-checked promptly rather than suppressed
> indefinitely. force_scan walks bypass the PUD test, so manual aging and
> newly added mm's always rescan and re-populate the filter.
> 
> To keep hot regions marked, also report the covering PUD from the rmap
> feedback path (lru_gen_look_around()), so regions whose hotness is only
> observed by eviction will be re-scanned.
> 
> Signed-off-by: Baoquan He <hebaoquan@kylinos.cn>
> ---
>  mm/vmscan.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 47 insertions(+), 5 deletions(-)
> 

...

> @@ -4397,8 +4431,14 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
>  	lazy_mmu_mode_disable();
>  
>  	/* feedback from rmap walkers to page table walkers */
> -	if (mm_state && suitable_to_scan(i, young))
> +	if (mm_state && suitable_to_scan(i, young)) {
> +		/* the PUD entry covering the young PTEs scanned above */
> +		pud_t *pud_p = pud_offset(p4d_offset(pgd_offset(vma->vm_mm, pvmw->address),
> +						     pvmw->address), pvmw->address);
> +
>  		update_pmd_bloom_filter(mm_state, max_seq, pvmw->pmd);
> +		update_pud_bloom_filter(mm_state, max_seq, pud_p);
> +	}

I saw the discussion from Barry about the concern about the overhead

I think we can drop the PUD ptr walk entirely. The filter only
needs a stable identity for the range walk_pud_range() is iterating,
and (mm, PUD-aligned address) gives that on both sides:

   walk_pud_range(), test and update alike:
      (void *)((unsigned long)args->mm ^ (addr & PUD_MASK))

   lru_gen_look_around():
      (void *)((unsigned long)vma->vm_mm ^ (pvmw->address & PUD_MASK))

Will this work? Also I took a look at the hash algorithm of this too:

static void get_item_key(void *item, int *key)
{
	u32 hash = hash_ptr(item, BLOOM_FILTER_SHIFT * 2);

	BUILD_BUG_ON(BLOOM_FILTER_SHIFT * 2 > BITS_PER_TYPE(u32));

	key[0] = hash & (BIT(BLOOM_FILTER_SHIFT) - 1);
	key[1] = hash >> BLOOM_FILTER_SHIFT;
}

And hash_ptr is actually hash_long, 64 bits if we use hash_32
will it be even better?