Re: [PATCH 3/6] mm/mglru: enhance cold/hot inversion handling in inc_min_seq()

[Baoquan He <baoquan.he@linux.dev>]

On 08/21/26 at 06:25pm, 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().

While sort_folio() move protected folio to the head of next gen too.
It only moves ineligible folios to the tail of next gen.

> 
> 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@kernel.org>
> ---
>  mm/vmscan.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 99ee3c833d54..3b618a51cde2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3946,9 +3946,12 @@ 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]);
> +			} else {
> +				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);
> -- 
> 2.34.1
>