Re: [PATCH v2 1/4] mm/vmscan: 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>
>
> As Qi mentioned [1], when swappiness=max (SWAPPINESS_ANON_ONLY) is set,
> the reclaim logic is expected to reclaim anonymous pages exclusively.
> However, due to the current ordering of checks in get_scan_count(),
> file pages may still be evicted if can_reclaim_anon_pages() returns
> false, which contradicts the semantics of SWAPPINESS_ANON_ONLY.
>
> Reproducer in a cgroup holding 64M of file cache, with no swap configured:
>
>   Before (file cache is wrongly evicted):
>     # cat memory.stat
>     anon 196608
>     file 67178496
>     pgscan_proactive 0
>     # echo "64M swappiness=max" > memory.reclaim
>     # cat memory.stat
>     anon 208896
>     file 4096                 <- page cache evicted
>     pgsteal_proactive 16400
>     pgscan_proactive 16400
>
>   After (file cache is left intact):
>     # cat memory.stat
>     anon 200704
>     file 67178496
>     pgscan_proactive 0
>     # echo "64M swappiness=max" > memory.reclaim
>     -bash: echo: write error: Resource temporarily unavailable
>     # cat memory.stat
>     anon 208896
>     file 67178496             <- page cache untouched
>     pgsteal_proactive 0
>     pgscan_proactive 0
>
> Fix this by bailing out early when SWAPPINESS_ANON_ONLY is set and no
> anonymous pages are reclaimable, before falling back to file reclaim.
>
> [1] https://lore.kernel.org/cgroups/7ddf3eee-5fe2-45f7-8614-c8936a039e04@xxxxxxxxx/
>
> Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
> Suggested-by: Qi Zheng <qi.zheng@xxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Reviewed-by: Muchun Song <muchun.song@xxxxxxxxx>
> Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>

Reviewed-by: Barry Song <baohua@xxxxxxxxxx>

> ---
>  mm/vmscan.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 35c3bb15ae96..d6b383d96a0c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2501,6 +2501,17 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>         enum scan_balance scan_balance;
>         enum lru_list lru;
>
> +       /* Proactive reclaim initiated by userspace for anonymous memory only */
> +       if (swappiness == SWAPPINESS_ANON_ONLY) {
> +               WARN_ON_ONCE(!sc->proactive);

I feel it's a bit odd that the WARN_ON_ONCE() is placed here. Maybe
adding a short comment would make the intent clearer. For example:
"SWAPPINESS_ANON_ONLY is only allowed for proactive reclaim."

I'm not sure whether it would be better to issue this warning earlier,
for example in try_to_free_mem_cgroup_pages(). That said, I don't
feel strongly about this.