Re: [PATCH v2 4/4] mm/mglru: fix anon-only reclaim evicting file pages when swappiness=max

[Barry Song]

On Sat, Jul 18, 2026 at 5:54 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
>
> From: Ridong Chen <chenridong@xxxxxxxxxx>
>
> The previous patch fixes this issue for the traditional LRU, which also
> exists in MGLRU [1]. Fix this by checking whether swappiness is
> SWAPPINESS_ANON_ONLY in get_swappiness() first, and returning 0 from
> get_nr_to_scan() when swappiness is SWAPPINESS_ANON_ONLY and anon pages
> cannot be reclaimed, to avoid useless work.

Is this intended to avoid unnecessary work, or to prevent file folios
from being reclaimed incorrectly?

>
> The test result:
> Before fix:
>
>   # cat /sys/kernel/mm/lru_gen/enabled
>   0x0007
>   # cat memory.stat
>   anon 204800
>   file 67108864
>   ...
>   pgsteal_proactive 0
>   pgscan_proactive 0
>
>   # echo "64M swappiness=max" > memory.reclaim
>   # cat memory.stat
>   anon 208896
>   file 0
>   ...
>   pgsteal_proactive 16384
>   pgscan_proactive 16384
>
> After fix:
>
>   # cat memory.stat
>   anon 188416
>   file 67215360
>   kernel 1970176
>   ...
>   pgsteal_proactive 0
>   pgscan_proactive 0
>
>   # echo "64M swappiness=max" > memory.reclaim
>   -bash: echo: write error: Resource temporarily unavailable
>   # cat memory.stat
>   anon 204800
>   file 67215360
>   ...
>   pgsteal_proactive 0
>   pgscan_proactive 0
>
> [1] https://sashiko.dev/#/patchset/20260717113300.214717-1-ridong.chen@xxxxxxxxx
>
> Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
[...]
>  static int get_nr_gens(struct lruvec *lruvec, int type)
> @@ -4915,6 +4919,14 @@ static long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *sc,
>                            struct mem_cgroup *memcg, int swappiness)
>  {
>         unsigned long nr_to_scan, evictable;
> +       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +
> +       /* Proactive reclaim: anon-only requested but anon is not reclaimable */
> +       if (swappiness == SWAPPINESS_ANON_ONLY &&
> +           !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
> +               WARN_ON_ONCE(!sc->proactive);

Similar to the previous patch, maybe it would help to add a bit more
comment here?

Thanks
Barry