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

[Lorenzo Stoakes <ljs@kernel.org>]

On Thu, Apr 09, 2026 at 09:06:08AM +0200, 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.

Agreed, there is still _huge_ confusion out there as to what this is for.

(If I hear another person tell me this is a way to 'free up memory' I'll scream
:)

Really I think it should be seen as a legacy thing for, as Michal says,
debug purposes (it would have been a good idea to put this in debugfs tbh).

And adding something to sysfs as a permanent, maintained interface for this
purpose seems problematic.

What is the use-case for this? Why are you dropping the caches? Presumably
for debugging/perf measuring/etc. purposes? The cover letter is lacking
that.

I wonder, if this is something we should move forward with, whether we
might be better off putting it in debugfs as a result?

>
> > 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?
>
> --
> Michal Hocko
> SUSE Labs

Thanks, Lorenzo