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

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

On 7/16/26 7:17 PM, Ridong Chen wrote:
> 
> 
> 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?
>

Sure, thanks for your time on this, and I look forward to your fix!