mm: memcg: deprecate the non-hierarchical mode

Patch series "mm: memcg: deprecate cgroup v1 non-hierarchical mode", v1.

The non-hierarchical cgroup v1 mode is a legacy of early days
of the memory controller and doesn't bring any value today.
However, it complicates the code and creates many edge cases
all over the memory controller code.

It's a good time to deprecate it completely. This patchset removes
the internal logic, adjusts the user interface and updates
the documentation. The alt patch removes some bits of the cgroup
core code, which become obsolete.

Michal Hocko said:
  "All that we know today is that we have a warning in place to complain
   loudly when somebody relies on use_hierarchy=0 with a deeper
   hierarchy. For all those years we have seen _zero_ reports that would
   describe a sensible usecase.

   Moreover we (SUSE) have backported this warning into old distribution
   kernels (since 3.0 based kernels) to extend the coverage and didn't
   hear even for users who adopt new kernels only very slowly. The only
   report we have seen so far was a LTP test suite which doesn't really
   reflect any real life usecase"

This patch (of 3):

The non-hierarchical cgroup v1 mode is a legacy of early days of the
memory controller and doesn't bring any value today.  However, it
complicates the code and creates many edge cases all over the memory
controller code.

It's a good time to deprecate it completely.

Functionally this patch enabled is by default for all cgroups and forbids
switching it off.  Nothing changes if cgroup v2 is used: hierarchical mode
was enforced from scratch.

To protect the ABI memory.use_hierarchy interface is preserved with a
limited functionality: reading always returns "1", writing of "1" passes
silently, writing of any other value fails with -EINVAL and a warning to
dmesg (on the first occasion).

Link: https://lkml.kernel.org/r/20201110220800.929549-1-guro@fb.com
Link: https://lkml.kernel.org/r/20201110220800.929549-2-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 5 deletions

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e41c21819ba0..80c5c34416e8 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -281,9 +281,6 @@ bool cgroup_ssid_enabled(int ssid)
  * - cpuset: a task can be moved into an empty cpuset, and again it takes
  *   masks of ancestors.
  *
- * - memcg: use_hierarchy is on by default and the cgroup file for the flag
- *   is not created.
- *
  * - blkcg: blk-throttle becomes properly hierarchical.
  *
  * - debug: disallowed on the default hierarchy.
@@ -5156,8 +5153,6 @@ static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
 	    cgroup_parent(parent)) {
 		pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
 			current->comm, current->pid, ss->name);
-		if (!strcmp(ss->name, "memory"))
-			pr_warn("\"memory\" requires setting use_hierarchy to 1 on the root\n");
 		ss->warned_broken_hierarchy = true;
 	}