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

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

On 4/9/2026 3:06 PM, Michal Hocko wrote:
> On Thu 09-04-26 14:35:03, Kefeng Wang wrote:
>> Add a sysfs interface at /sys/devices/system/node/nodeX/drop_caches
>> to allow dropping caches on a specific NUMA node.
>>
>> The existing global drop_caches mechanism (/proc/sys/vm/drop_caches)
>> operates across all NUMA nodes indiscriminately, causing,
>> - Unnecessary eviction of hot cache on some nodes
>> - Performance degradation for applications with NUMA affinity
>> - Long times spent on large systems with lots of memory
>>
>> By exposing a per-node interface, admistrator can,
>> - Target specific nodes experiencing memory pressure
>> - Preserve cache on unaffected nodes
>> - Perform reclamation with finer granularity
> 
> Quite honestly drop_caches is not the best interface to build any new
> functionality on top of. It has backfired a lot in the past and we have
> tried to make it extra clear that this should be used for debugging
> purposes only. Extending it further sounds like a bad step.


I am not clear about this history of this interface, but some of our
products do use this interface :(

> 
>> One use cases is hot-pluggable NUMA nodes, during hot-remove, simply
>> dropping pagecache is far more efficient than migrating large amounts
>> of pages to other nodes, which also eliminating the risk of accessing
>> potentially faulty memory.
> 
> Does the per-node reclaim interface can help with this by
> any means?

I have also considered this interface. As mentioned above, our product
uses drop_caches, so it is easy to convert to using the per-node
drop_caches interface.

'echo 1 > per-node-dropcaches'  VS 'echo XG swappiness=0 > per-node-reclaim'

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,

So if per-node-drop_caches cannot be accepted, we can consider expanding
the reclaim interface to reclaim file pages?