Re: [PATCH v7 mm-unstable] mm: vmscan: retry folios written back while isolated for traditional LRU

[Johannes Weiner <hannes@cmpxchg.org>]

On Sat, Jan 11, 2025 at 09:15:04AM +0000, Chen Ridong wrote:
> @@ -5706,6 +5706,44 @@ static void lru_gen_shrink_node(struct pglist_data *pgdat, struct scan_control *
>  
>  #endif /* CONFIG_LRU_GEN */
>  
> +/**
> + * find_folios_written_back - Find and move the written back folios to a new list.
> + * @list: filios list
> + * @clean: the written back folios list
> + * @lruvec: the lruvec
> + * @type: LRU_GEN_ANON/LRU_GEN_FILE, only for multi-gen LRU
> + * @skip_retry: whether skip retry.
> + */
> +static inline void find_folios_written_back(struct list_head *list,
> +		struct list_head *clean, struct lruvec *lruvec, int type, bool skip_retry)
> +{
> +	struct folio *folio;
> +	struct folio *next;
> +
> +	list_for_each_entry_safe_reverse(folio, next, list, lru) {
> +#ifdef CONFIG_LRU_GEN
> +		DEFINE_MIN_SEQ(lruvec);
> +#endif
> +		if (!folio_evictable(folio)) {
> +			list_del(&folio->lru);
> +			folio_putback_lru(folio);
> +			continue;
> +		}
> +
> +		/* retry folios that may have missed folio_rotate_reclaimable() */
> +		if (!skip_retry && !folio_test_active(folio) && !folio_mapped(folio) &&
> +		    !folio_test_dirty(folio) && !folio_test_writeback(folio)) {
> +			list_move(&folio->lru, clean);
> +			continue;
> +		}
> +#ifdef CONFIG_LRU_GEN
> +		/* don't add rejected folios to the oldest generation */
> +		if (lru_gen_enabled() && lru_gen_folio_seq(lruvec, folio, false) == min_seq[type])
> +			set_mask_bits(&folio->flags, LRU_REFS_FLAGS, BIT(PG_active));
> +#endif
> +	}

Can't this solved much more easily by acting on the flag in the
generic LRU add/putback path? Instead of walking the list again.

Especially with Kirill's "[PATCH 0/8] mm: Remove PG_reclaim" that
removes the PG_readahead ambiguity.