Re: [PATCH v4 3/3] mm: zswap: drop cold writeback folios via swap dropbehind

[Kunwu Chan <kunwu.chan@gmail.com>]

On Tue, 25 Aug 2026 15:52:07 +0200 Alexandre Ghiti <alex@ghiti.fr> wrote:

> zswap writeback decompresses an entry into a fresh swap cache folio and
> writes it back. The folio is cold by construction, yet it is left on the
> LRU for reclaim to find and free later, wasting a reclaim scan and keeping
> cold memory resident longer than necessary.
> 
> Allocate the folio off the LRU and mark it PG_dropbehind so the swap
> dropbehind path frees it from the swap cache once writeback completes.
> 
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Suggested-by: Nhat Pham <nphamcs@gmail.com>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>  mm/zswap.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 8163e6c5f76c..d16822a516e8 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1013,7 +1013,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>  	 */
>  	if (IS_ERR(folio))
>  		return PTR_ERR(folio);
> -	folio_add_lru(folio);
>  
>  	/*
>  	 * folio is locked, and the swapcache is now secured against
> @@ -1046,12 +1045,26 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>  	/* folio is up to date */
>  	folio_mark_uptodate(folio);
>  
> -	/* move it to the tail of the inactive list after end_writeback */
> -	folio_set_reclaim(folio);
> +	folio_set_dropbehind(folio);
> +
> +	/*
> +	 * Drop our reference before starting writeback so the swap cache holds
> +	 * the only one: the drop in folio_end_writeback() needs that for
> +	 * remove_mapping_reclaim() to succeed, otherwise the folio is handed
> +	 * back to reclaim instead.
> +	 *
> +	 * Nothing can free the folio in the meantime: we hold the folio lock
> +	 * until writeback starts, PG_writeback then blocks swap cache removal,
> +	 * and folio_end_writeback() takes its own reference before clearing
> +	 * PG_writeback and donates it to the drop.
> +	 */
> +	folio_put(folio);
>  
>  	/* start writeback */
>  	__swap_writepage(folio, NULL);

Thanks for addressing this.

I checked the refcount handoff in v4. Dropping the zswap reference before 
__swap_writepage() removes the refcount overlap I was concerned about in v3. 
The swapcache reference keeps the folio alive until writeback starts, and 
folio_end_writeback() takes the completion reference before clearing 
PG_writeback, so remove_mapping_reclaim() sees the expected refcount.

Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>

Thanks, 
KunWu

>  
> +	return 0;
> +
>  out:
>  	if (ret) {
>  		swap_cache_del_folio(folio);
> -- 
> 2.53.0-Meta
> 
>