Re: [PATCH] mm/memcontrol: Add the drop_cache interface for cgroup v2

From: Chris Down
Date: Mon Sep 21 2020 - 06:43:11 EST

Michal Hocko writes:
On Mon 21-09-20 16:02:55, zangchunxin@xxxxxxxxxxxxx wrote:
From: Chunxin Zang <zangchunxin@xxxxxxxxxxxxx>

In the cgroup v1, we have 'force_mepty' interface. This is very
useful for userspace to actively release memory. But the cgroup
v2 does not.

This patch reuse cgroup v1's function, but have a new name for
the interface. Because I think 'drop_cache' may be is easier to
understand :)

This should really explain a usecase. Global drop_caches is a terrible
interface and it has caused many problems in the past. People have
learned to use it as a remedy to any problem they might see and cause
other problems without realizing that. This is the reason why we even
log each attempt to drop caches.

I would rather not repeat the same mistake on the memcg level unless
there is a very strong reason for it.

I agree with Michal. We already have ways to do best-effort memory release on cgroup v2, primarily with memory.high. Singling out a category of memory for reclaim has historically proved to be a fool's errand.