[PATCH 0/8] memcg: separate private and public ID namespaces

Shakeel Butt
2025-12-25T23:21:24Z

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

Note: please apply this series after the patch at
https://lore.kernel.org/20251225002904.139543-1-shakeel.butt@linux.dev/

Shakeel Butt (8):
memcg: introduce private id API for in-kernel users
memcg: expose mem_cgroup_ino() and mem_cgroup_get_from_ino()
unconditionally
memcg: mem_cgroup_get_from_ino() returns NULL on error
memcg: use cgroup_id() instead of cgroup_ino() for memcg ID
mm/damon: use cgroup ID instead of private memcg ID
mm/vmscan: use cgroup ID instead of private memcg ID in lru_gen
interface
memcg: remove unused mem_cgroup_id() and mem_cgroup_from_id()
memcg: rename mem_cgroup_ino() to mem_cgroup_id()

include/linux/damon.h | 4 +--
include/linux/memcontrol.h | 26 +++++++----------
mm/damon/core.c | 7 ++---
mm/damon/sysfs-schemes.c | 6 ++--
mm/list_lru.c | 2 +-
mm/memcontrol-v1.c | 6 ++--
mm/memcontrol-v1.h | 4 +--
mm/memcontrol.c | 60 ++++++++++++++++++--------------------
mm/shrinker_debug.c | 13 +++++----
mm/vmscan.c | 17 ++++-------
mm/workingset.c | 8 ++---
11 files changed, 68 insertions(+), 85 deletions(-)

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2.47.3