Re: [PATCH] mm/mglru: Fix young counter undercount for large folios

[Baolin Wang]

On 8/12/26 2:59 PM, Hui Zhu wrote:
> From: Hui Zhu <zhuhui@xxxxxxxxxx>
>
> In lru_gen_look_around(), the young counter tracks the number of young
> PTEs.  The original folio's contribution is represented by the initial
> value of young: test_and_clear_young_ptes_notify() is called on it at
> function entry, and the function returns early if it is not young.  In
> the subsequent loop, the original folio is skipped (its accessed bits
> were already cleared), so it is not double-counted.
>
> However, young is initialized to 1 regardless of the folio size.  When
> the original folio is a large folio with nr PTEs, its young count is
> underestimated by nr - 1.  This inconsistency can cause
> suitable_to_scan() to return false, preventing the PMD from being added
> to the bloom filter and reducing aging accuracy for mTHP workloads.
>
> Initialize young to nr so the original folio is accounted the same way
> as other young folios in the loop (young += nr).
>
> Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> ---

Good catch. Please also add the Fixes tag:

Fixes: 56e5b60b2114 ("mm: support batched checking of the young flag for 
MGLRU")

With that,
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

>   mm/vmscan.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc324e37c5f1..264017850a55 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4192,7 +4192,7 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
>   	unsigned long end;
>   	struct lru_gen_mm_walk *walk;
>   	struct folio *last = NULL;
> -	int young = 1;
> +	int young = nr;
>   	pte_t *pte = pvmw->pte;
>   	unsigned long addr = pvmw->address;
>   	struct vm_area_struct *vma = pvmw->vma;