Re: [PATCH v2 3/7] mm/mglru: enhance cold/hot inversion handling in inc_min_seq()

[Ridong Chen]

On 8/28/2026 7:47 AM, Barry Song (Xiaomi) wrote:
> During aging, a folio's generation may already have been updated by
> folio_update_gen(), even though it has not yet been moved to the
> corresponding generation list. Such folios are hotter than those
> already in that generation.
>
> It makes sense for inc_min_seq() to increment the generation of
> folios that were never promoted during aging and move them to the
> tail of the new oldest generation. However, folios that were already
> promoted should instead be moved to the head of their updated
> generation, just as sort_folio() does in scan_folios().
>
> Otherwise, promoted folios could end up behind folios that were
> never promoted, effectively inverting their hot/cold ordering.
>
> Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>
> Tested-by: Xueyuan Chen <xueyuan.chen21@xxxxxxxxx>
> ---
>   mm/vmscan.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 07c22d51debd..b10d1703d907 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3949,9 +3949,12 @@ static bool inc_min_seq(struct lruvec *lruvec, int type, int swappiness)
>   			VM_WARN_ON_ONCE_FOLIO(folio_zonenum(folio) != zone, folio);
>   
>   			new_gen = __folio_inc_gen(folio, old_gen, &gen_increased);
> -			list_move_tail(&folio->lru, &lrugen->folios[new_gen][type][zone]);
> -			if (gen_increased)
> +			if (gen_increased) {
>   				delta += nr_pages;
> +				list_move_tail(&folio->lru, &lrugen->folios[new_gen][type][zone]);

Nit.
This is a bit confusing to me.

When I read the code, I thought this implementation contradicts what the commit 
message says, which means I thought that we move the folios that have been 
promoted to the tail.

> +			} else {

Maybe adding a comment would make it clearer, like:

If gen_increased == false, it means the folio has been promoted, so move it to 
the head.

> +				list_move(&folio->lru, &lrugen->folios[new_gen][type][zone]);
> +			}



>   			/* don't count the workingset being lazily promoted */
>   			if (refs + workingset != BIT(LRU_REFS_WIDTH) + 1) {
>   				int tier = lru_tier_from_refs(refs, workingset);

Overall, looks good to me.

Reviewed-by: Ridong Chen <ridong.chen@xxxxxxxxx>

--
Best regards
Ridong