Re: [PATCH v2 1/4] mm/vmscan: fix anon-only reclaim evicting file pages when swappiness=max

[Qi Zheng <qi.zheng@linux.dev>]

On 7/18/26 5:52 PM, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> 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@linux.dev/
> 
> Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim for only anon reclaim")
> Suggested-by: Qi Zheng <qi.zheng@linux.dev>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Reviewed-by: Muchun Song <muchun.song@linux.dev>
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
> ---
>   mm/vmscan.c | 18 +++++++++++-------
>   1 file changed, 11 insertions(+), 7 deletions(-)

Thanks for the fix!

Reviewed-by: Qi Zheng <qi.zheng@linux.dev>

> 
> 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);
> +		if (!can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
> +			memset(nr, 0, sizeof(*nr) * NR_LRU_LISTS);
> +			return;
> +		}
> +		scan_balance = SCAN_ANON;
> +		goto out;
> +	}
> +
>   	/* If we have no swap space, do not bother scanning anon folios. */
>   	if (!sc->may_swap || !can_reclaim_anon_pages(memcg, pgdat->node_id, sc)) {
>   		scan_balance = SCAN_FILE;
> @@ -2519,13 +2530,6 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>   		goto out;
>   	}
>   
> -	/* Proactive reclaim initiated by userspace for anonymous memory only */
> -	if (swappiness == SWAPPINESS_ANON_ONLY) {
> -		WARN_ON_ONCE(!sc->proactive);
> -		scan_balance = SCAN_ANON;
> -		goto out;
> -	}
> -
>   	/*
>   	 * Do not apply any pressure balancing cleverness when the
>   	 * system is close to OOM, scan both anon and file equally