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

[Ridong Chen <ridong.chen@linux.dev>]

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;
	}
	....
	/* 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;
	}

	...
}
```

-- 
Best regards
Ridong