Re: [PATCH RFC] fs: drop_caches: introduce per-node drop_caches interface

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 4/9/2026 8:50 PM, Kefeng Wang wrote:
> 
> 
> On 4/9/2026 6:52 PM, Michal Hocko wrote:
>> On Thu 09-04-26 16:54:48, Kefeng Wang wrote:
>>>
>>>
>>> On 4/9/2026 4:22 PM, Michal Hocko wrote:
>>>> On Thu 09-04-26 16:08:37, Kefeng Wang wrote:
>>>> [...]
>>>>> To use the reclaim interface, there are two differences,
>>>>>
>>>>> 1) we need to input the reclaim numbers and swappiness, this is not 
>>>>> a big
>>>>> problem
>>>>> 2) for reclaim, it supports swappiness=max to only reclaim 
>>>>> anonymous pages,
>>>>> but cannot only reclaim file pages,
>>>>
>>>> Why is 2) a real constrain?
>>>
>>> This should not be a restriction, but a strategy. Our product wants to
>>> migrate anonymous pages to the local instead of swapping them out. 
>>> However,
>>> the current per-node-reclaim interface does not support reclaiming 
>>> only file
>>> pages.
>>
>> Yes, I do understand that you want to keep your hot anonymous pages
>> resident on some node. Those shouldn't be reclaimed by the user space
>> triggered reclaim anyway, right? Migration will then happen during the
>> memory offlining.
> 
> Yes, that we need.
>>
>> This will certainly require some fine tuning but I do not see any reason
>> this should be completely impossible. Certainly a more robust way (from
>> API POV) than the suggested drop_caches. I am also not convinced we need
>> page-cache-only reclaim for the existing reclaim interface. I believe it
>> makes more sense to look at the reclaim from hotness POV rather than
>> anon vs. file.
> 
> It is already support only reclaim file pages when swappiness = 0 for
> memcg.reclaim, but not for per-node-reclaim, so we can just make a few
> small changes to enable it(no tested)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 84eba9ab5d25..46254ae9b8df 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2515,7 +2515,7 @@ static void get_scan_count(struct lruvec *lruvec, 
> struct scan_control *sc,
>           * using the memory controller's swap limit feature would be
>           * too expensive.
>           */
> -       if (cgroup_reclaim(sc) && !swappiness) {
> +       if ((cgroup_reclaim(sc) || sc->proactive) && !swappiness) {
>                  scan_balance = SCAN_FILE;
>                  goto out;
>          }
> 

Sorry, the changes below are incorrect，please ignore.

> or
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 84eba9ab5d25..e2998b61f78b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2515,7 +2515,7 @@ static void get_scan_count(struct lruvec *lruvec, 
> struct scan_control *sc,
>           * using the memory controller's swap limit feature would be
>           * too expensive.
>           */
> -       if (cgroup_reclaim(sc) && !swappiness) {
> +       if (sc->proactive && !swappiness) {
>                  scan_balance = SCAN_FILE;
>                  goto out;
>          }
> 
> 
> 
>