Re: [PATCH v9 3/6] mm: memcontrol: add interface for swap tier selection

[Yosry Ahmed]

On Sat, Jun 20, 2026 at 11:17 AM Youngjun Park <her0gyugyu@xxxxxxxxx> wrote:
>
> Introduce memory.swap.tiers.max, a flat-keyed file listing each
> tier defined in /sys/kernel/mm/swap/tiers with its state, "max"
> (allowed, the default) or "0" (disabled).  A tier is one bit in the
> cgroup's tier mask, so writing "<tier> max" or "<tier> 0" sets or
> clears that bit.
>
> Since the current use case lacks amount control, it only supports
> "max" (on) and "0" (off). Therefore, it does not track per-tier swap
> usage, relying instead on a fast runtime bitmask check.
>
> We maintain both `mask` and `effective_mask`. The `effective_mask` is
> strictly bounded by the parent (e.g., if a parent is "0", the child's
> effective state is "0" even if its `mask` is "max"). Maintaining this
> separately avoids costly cgroup tree traversals to check ancestors at
> runtime.
>
> Suggested-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Signed-off-by: Youngjun Park <youngjun.park@xxxxxxx>
> ---
>  Documentation/admin-guide/cgroup-v2.rst |  20 +++++
>  Documentation/mm/swap-tier.rst          |   9 +++
>  include/linux/memcontrol.h              |   5 ++
>  mm/memcontrol.c                         |  67 ++++++++++++++++
>  mm/swap_state.c                         |   5 +-
>  mm/swap_tier.c                          | 102 +++++++++++++++++++++++-
>  mm/swap_tier.h                          |  57 +++++++++++--
>  7 files changed, 255 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 6efd0095ed99..4843ffcfd110 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1850,6 +1850,26 @@ The following nested keys are defined.
>         Swap usage hard limit.  If a cgroup's swap usage reaches this
>         limit, anonymous memory of the cgroup will not be swapped out.
>
> +  memory.swap.tiers.max
> +       A read-write flat-keyed file which exists on non-root
> +       cgroups.  The default is "max" for every tier.

I wonder what should the default behavior be if memory.swap.max is set
to a value other than "max". Should the limits in
memory.swap.tiers.max auto-scale or remain as "max"? We probably want
to keep the behavior consistent with memory tiering.

Shakeel/Joshua, WDYT?