Re: [PATCH v2 2/2] mm: vmscan: fix node reclaim ignoring swappiness parameter

Hi Ridong,

Reproduced the bug and verified the fix in a QEMU x86_64 guest with
CONFIG_MEMCG disabled. Global vm.swappiness pinned to 1, then:

    echo "64M swappiness=<x>" > /sys/devices/system/node/node0/reclaim

  swappiness= | buggy (anon/file)   | patched (anon/file)
  ----------- | -------------------- | --------------------
  max         |      0  /  16385     |  16292  /  0
  200         |      2  /  16384     |  16837  /  0
  1           |      0  /  16386     |      0  /  16467

Unpatched ignores the swappiness arg (always file); patched honors it.
Matches the commit message.

Tested-by: Song Hu <[email protected]>
Reviewed-by: Song Hu <[email protected]>

在 2026/7/16 11:17, Ridong 写道:
> From: Ridong Chen <[email protected]>
> 
> sc_swappiness() had two separate definitions depending on
> CONFIG_MEMCG. The !CONFIG_MEMCG variant simply returned
> vm_swappiness, ignoring the proactive_swappiness value passed
> through scan_control. This caused the swappiness parameter
> written to /sys/devices/system/node/nodeX/reclaim to have no
> effect when CONFIG_MEMCG is disabled.
> 
> Fix this by consolidating sc_swappiness() into a single definition
> that checks sc->proactive_swappiness first, then falls back to
> mem_cgroup_swappiness() which already handles both CONFIG_MEMCG
> and !CONFIG_MEMCG.
> 
> Before fix (swappiness=max ignored, mostly file pages reclaimed):
> 
>     # cat /proc/sys/vm/swappiness
>     60
>     # cat /proc/vmstat | grep pgsteal
>     pgsteal_kswapd 0
>     pgsteal_direct 0
>     pgsteal_khugepaged 0
>     pgsteal_proactive 1840
>     pgsteal_anon 25
>     pgsteal_file 1815
>     # echo "64M swappiness=max" > /sys/devices/system/node/node0/reclaim
>     # cat /proc/vmstat | grep pgsteal
>     pgsteal_kswapd 0
>     pgsteal_direct 0
>     pgsteal_khugepaged 0
>     pgsteal_proactive 18013
>     pgsteal_anon 337
>     pgsteal_file 17676
> 
> After fix (swappiness=max honored, anon pages reclaimed as expected):
> 
>     # cat /proc/vmstat | grep pgsteal
>     pgsteal_kswapd 0
>     pgsteal_direct 0
>     pgsteal_khugepaged 0
>     pgsteal_proactive 0
>     pgsteal_anon 0
>     pgsteal_file 0
>     # echo "64M swappiness=max" > /sys/devices/system/node/node0/reclaim
>     # cat /proc/vmstat | grep pgsteal
>     pgsteal_kswapd 0
>     pgsteal_direct 0
>     pgsteal_khugepaged 0
>     pgsteal_proactive 16283
>     pgsteal_anon 16283
>     pgsteal_file 0
> 
> Cc: [email protected] # 6.17+
> Fixes: b980077899ea ("mm: introduce per-node proactive reclaim interface")
> Signed-off-by: Ridong Chen <[email protected]>
> Acked-by: Johannes Weiner <[email protected]>
> Reviewed-by: Barry Song <[email protected]>
> ---
>  mm/vmscan.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 986dde8e7429..bd71595c8b2b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -199,6 +199,13 @@ struct scan_control {
>   */
>  int vm_swappiness = 60;
>  
> +static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> +{
> +	if (sc->proactive && sc->proactive_swappiness)
> +		return *sc->proactive_swappiness;
> +	return mem_cgroup_swappiness(memcg);
> +}
> +
>  #ifdef CONFIG_MEMCG
>  
>  /* Returns true for reclaim through cgroup limits or cgroup interfaces. */
> @@ -239,13 +246,6 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  #endif
>  	return false;
>  }
> -
> -static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> -{
> -	if (sc->proactive && sc->proactive_swappiness)
> -		return *sc->proactive_swappiness;
> -	return mem_cgroup_swappiness(memcg);
> -}
>  #else
>  static bool cgroup_reclaim(struct scan_control *sc)
>  {
> @@ -261,11 +261,6 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  {
>  	return true;
>  }
> -
> -static int sc_swappiness(struct scan_control *sc, struct mem_cgroup *memcg)
> -{
> -	return READ_ONCE(vm_swappiness);
> -}
>  #endif
>  
>  static void set_task_reclaim_state(struct task_struct *task,