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

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

On 7/16/2026 4:42 PM, Qi Zheng wrote:
> 
> 
> 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.
> 

Much better.
Would you mind if I send a separate patch to fix this issue?

> 
>>      }
>>      ....
>>      /* 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