memcg: introduce private id API for in-kernel users

Patch series "memcg: separate private and public ID namespaces".

The memory cgroup subsystem maintains a private ID infrastructure that
is decoupled from the cgroup IDs. This private ID system exists because
some kernel objects (like swap entries and shadow entries in the
workingset code) can outlive the cgroup they were associated with.
The motivation is best described in commit 73f576c04b941 ("mm:
memcontrol: fix cgroup creation failure after many small jobs").

Unfortunately, some in-kernel users (DAMON, LRU gen debugfs interface,
shrinker debugfs) started exposing these private IDs to userspace.
This is problematic because:

1. The private IDs are internal implementation details that could change
2. Userspace already has access to cgroup IDs through the cgroup
   filesystem
3. Using different ID namespaces in different interfaces is confusing

This series cleans up the memcg ID infrastructure by:

1. Explicitly marking the private ID APIs with "private" in their names
   to make it clear they are for internal use only (swap/workingset)

2. Making the public cgroup ID APIs (mem_cgroup_id/mem_cgroup_get_from_id)
   unconditionally available

3. Converting DAMON, LRU gen, and shrinker debugfs interfaces to use
   the public cgroup IDs instead of the private IDs

4. Removing the now-unused wrapper functions and renaming the public
   APIs for clarity

After this series:
- mem_cgroup_private_id() / mem_cgroup_from_private_id() are used for
  internal kernel objects that outlive their cgroup (swap, workingset)
- mem_cgroup_id() / mem_cgroup_get_from_id() return the public cgroup ID
  (from cgroup_id()) for use in userspace-facing interfaces


This patch (of 8):

The memory cgroup maintains a private ID infrastructure decoupled from the
cgroup IDs for swapout records and shadow entries.  The main motivation of
this private ID infra is best described in the commit 73f576c04b941 ("mm:
memcontrol: fix cgroup creation failure after many small jobs").

Unfortunately some users have started exposing these private IDs to the
userspace where they should have used the cgroup IDs which are already
exposed to the userspace.  Let's rename the memcg ID APIs to explicitly
mark them private.

No functional change is intended.

Link: https://lkml.kernel.org/r/20251225232116.294540-1-shakeel.butt@linux.dev
Link: https://lkml.kernel.org/r/20251225232116.294540-2-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 21 insertions, 3 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index fd400082313a..1c4224bcfb23 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -65,7 +65,7 @@ struct mem_cgroup_reclaim_cookie {
 
 #define MEM_CGROUP_ID_SHIFT	16
 
-struct mem_cgroup_id {
+struct mem_cgroup_private_id {
 	int id;
 	refcount_t ref;
 };
@@ -191,7 +191,7 @@ struct mem_cgroup {
 	struct cgroup_subsys_state css;
 
 	/* Private memcg ID. Used to ID objects that outlive the cgroup */
-	struct mem_cgroup_id id;
+	struct mem_cgroup_private_id id;
 
 	/* Accounted resources */
 	struct page_counter memory;		/* Both v1 & v2 */
@@ -821,13 +821,19 @@ void mem_cgroup_iter_break(struct mem_cgroup *, struct mem_cgroup *);
 void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
 			   int (*)(struct task_struct *, void *), void *arg);
 
-static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
+static inline unsigned short mem_cgroup_private_id(struct mem_cgroup *memcg)
 {
 	if (mem_cgroup_disabled())
 		return 0;
 
 	return memcg->id.id;
 }
+struct mem_cgroup *mem_cgroup_from_private_id(unsigned short id);
+
+static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
+{
+	return mem_cgroup_private_id(memcg);
+}
 struct mem_cgroup *mem_cgroup_from_id(unsigned short id);
 
 #ifdef CONFIG_SHRINKER_DEBUG
@@ -1290,6 +1296,18 @@ static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
 	return NULL;
 }
 
+static inline unsigned short mem_cgroup_private_id(struct mem_cgroup *memcg)
+{
+	return 0;
+}
+
+static inline struct mem_cgroup *mem_cgroup_from_private_id(unsigned short id)
+{
+	WARN_ON_ONCE(id);
+	/* XXX: This should always return root_mem_cgroup */
+	return NULL;
+}
+
 #ifdef CONFIG_SHRINKER_DEBUG
 static inline unsigned long mem_cgroup_ino(struct mem_cgroup *memcg)
 {