This patchset adds a memory.lru_gen interface to cgroup v2, allowing users
to interact with MGLRU directly on a specific cgroup without needing to
know the internal memcg_id.

Motivation
==========
Currently, the only way to perform aging or eviction on a specific memcg
is through the debugfs interface (/sys/kernel/debug/lru_gen), which
requires the memcg_id. However, there's no straightforward way to get the
memcg_id for a given cgroup path. This makes it difficult for users to
leverage MGLRU's proactive reclaim capabilities on specific cgroups.

Solution
========
The new memory.lru_gen interface operates directly on the cgroup:

  # Show lru_gen info for this cgroup
  cat /sys/fs/cgroup/mygroup/memory.lru_gen

  # Run aging on node 0
  echo '+ 0 <seq>' > /sys/fs/cgroup/mygroup/memory.lru_gen

  # Evict cold pages on node 0
  echo '- 0 <seq> <swappiness> <nr_to_reclaim>' > \
       /sys/fs/cgroup/mygroup/memory.lru_gen

This interface is available on all cgroups including root, providing the
same functionality as the debugfs lru_gen interface.

Testing
=======
Create 1GB page cache, loop access 200MB as hot pages.
After aging twice, 200MB hot pages are in young generation,
800MB cold pages remain in oldest generation.
Eviction with seq=min_seq only reclaims cold pages, hot pages preserved.

Patches
=======
Patch 1 refactors the existing debugfs code to extract helper functions.
Patch 2 adds the memory.lru_gen interface using these helpers.
Patch 3 adds documentation for the new interface.


Jiayuan Chen (3):
  mm/lru_gen: refactor to extract helper functions
  mm/lru_gen: add memory.lru_gen interface for cgroup v2
  docs/cgroup: document memory.lru_gen interface

 Documentation/admin-guide/cgroup-v2.rst |  17 +++
 include/linux/mmzone.h                  |  16 +++
 mm/memcontrol.c                         |  31 +++++
 mm/vmscan.c                             | 176 +++++++++++++++++++-----
 4 files changed, 206 insertions(+), 34 deletions(-)

-- 
2.43.0

On Wed, Jan 21, 2026 at 08:39:46PM +0800, Jiayuan Chen wrote:
> This patchset adds a memory.lru_gen interface to cgroup v2, allowing users
> to interact with MGLRU directly on a specific cgroup without needing to
> know the internal memcg_id.

Unfortunetely we don't want to expose reclaim implementation specific
interface to a cgroup. 

> 
> Motivation
> ==========
> Currently, the only way to perform aging or eviction on a specific memcg
> is through the debugfs interface (/sys/kernel/debug/lru_gen), which
> requires the memcg_id. However, there's no straightforward way to get the
> memcg_id for a given cgroup path. This makes it difficult for users to
> leverage MGLRU's proactive reclaim capabilities on specific cgroups.

From the next kernel version, this memcg_id will be inode number of the
cgroup for this interface. So, a simple 'ls -id cgroup_path' would  be
sufficient for your use-case.

The relevant series [1] is in mm-tree at the moment and if you want, you
can backport it to your kernels.

[1] https://lkml.kernel.org/r/20251225232116.294540-1-shakeel.butt@linux.dev

January 22, 2026 at 06:19, "Shakeel Butt" <shakeel.butt@linux.dev mailto:shakeel.butt@linux.dev?to=%22Shakeel%20Butt%22%20%3Cshakeel.butt%40linux.dev%3E > wrote:


> 
> On Wed, Jan 21, 2026 at 08:39:46PM +0800, Jiayuan Chen wrote:
> 
> > 
> > This patchset adds a memory.lru_gen interface to cgroup v2, allowing users
> >  to interact with MGLRU directly on a specific cgroup without needing to
> >  know the internal memcg_id.
> > 
> Unfortunetely we don't want to expose reclaim implementation specific
> interface to a cgroup. 
> 
> > 
> > Motivation
> >  ==========
> >  Currently, the only way to perform aging or eviction on a specific memcg
> >  is through the debugfs interface (/sys/kernel/debug/lru_gen), which
> >  requires the memcg_id. However, there's no straightforward way to get the
> >  memcg_id for a given cgroup path. This makes it difficult for users to
> >  leverage MGLRU's proactive reclaim capabilities on specific cgroups.
> > 
> From the next kernel version, this memcg_id will be inode number of the
> cgroup for this interface. So, a simple 'ls -id cgroup_path' would be
> sufficient for your use-case.
> 
> The relevant series [1] is in mm-tree at the moment and if you want, you
> can backport it to your kernels.
> 
> [1] https://lkml.kernel.org/r/20251225232116.294540-1-shakeel.butt@linux.dev
>

Hi Shakeel,

Thanks for the review and the pointer to the inode-based memcg_id series.

I agree that using the cgroup inode number as memcg_id will simplify the
write operations (aging/eviction) through the debugfs interface.

However, I'd like to point out that the read operation (viewing lru_gen
info for a specific cgroup) is still not convenient. Currently, users
would need to parse the full debugfs output and grep for the specific
memcg_id, which can be cumbersome especially on systems with many cgroups.

Would it be acceptable to add a read-only command to /lru_gen that only displays
the lru_gen information for the specified cgroup?

Alternatively, if exposing any lru_gen info in cgroup is not desired, I
understand and will use the debugfs approach with scripting.

Thanks,
chenjiayuan