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

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

Hi Ridong,

On 7/16/26 11:17 AM, Ridong wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> 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):
> 

By the way, in get_scan_count(), it seems we still reclaim file pages
without swap space, even if swappiness=max (SWAPPINESS_ANON_ONLY) is
set.

This behavior appears to contradict the semantics of
SWAPPINESS_ANON_ONLY, which probably needs a fix as well.

Thanks,
Qi