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

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

On 7/16/26 4:21 PM, Ridong Chen wrote:
> 
> 
> On 7/16/2026 11:58 AM, Qi Zheng wrote:
>> 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.
> 
> You're right. Currently, when can_reclaim_anon_pages() returns false, 
> the reclaim logic falls back to scanning file folios even if swappiness 
> == SWAPPINESS_ANON_ONLY is set:
> 
> ```
> static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>                 unsigned long *nr)
> {
>      ....
>      /* 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;
>          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;
>      }
>      ...
> }
> ```
> 
> The !can_reclaim_anon_pages() check above takes precedence over the 
> SWAPPINESS_ANON_ONLY branch, making the latter effectively unreachable 
> when anonymous pages are deemed non-reclaimable.
> 
> To fix this, could we move the SWAPPINESS_ANON_ONLY check up before the 
> can_reclaim_anon_pages() check?
> 
> ```
> static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>                 unsigned long *nr)
> {
>      ...
>      /* 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;

For SWAPPINESS_ANON_ONLY + !can_reclaim_anon_pages, perhaps we should
just return directly instead of goto out, since no pages can be
reclaimed in this situation.

Thanks,
Qi

>      }
>      ....
>      /* 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;
>          goto out;
>      }
> 
>      ...
> }
> ```
>