linux-kernel - Re: [RFC PATCH v3 3/5] mm: memcontrol: add interface for swap tier selection

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Message-ID: <ixlef27mi6vm5pek775kyddai7rkzls6mjo434rvwwp5gulcp5@n3uzy35ta7me>
Date: Tue, 3 Feb 2026 11:54:41 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Youngjun Park <youngjun.park@....com>
Cc: akpm@...ux-foundation.org, chrisl@...nel.org, kasong@...cent.com, 
	hannes@...xchg.org, mhocko@...nel.org, roman.gushchin@...ux.dev, 
	shakeel.butt@...ux.dev, muchun.song@...ux.dev, shikemeng@...weicloud.com, 
	nphamcs@...il.com, bhe@...hat.com, baohua@...nel.org, cgroups@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, gunho.lee@....com, 
	taejoon.song@....com
Subject: Re: [RFC PATCH v3 3/5] mm: memcontrol: add interface for swap tier
 selection

Hi.

This is merely the API feedback.

(Feedback to the propsed form, I'm not sure whether/how this should
interact with memory.swap.max (formally cf io.weight).)

On Sat, Jan 31, 2026 at 09:54:52PM +0900, Youngjun Park <youngjun.park@....com> wrote:
> This patch integrates the swap tier infrastructure with cgroup,
> enabling the selection of specific swap devices per cgroup by
> configuring allowed swap tiers.
> 
> The new `memory.swap.tiers` interface controls allowed swap tiers via a mask.
> By default, the mask is set to include all tiers, allowing specific tiers to
> be excluded or restored. Note that effective tiers are calculated separately
> using a dedicated mask to respect the cgroup hierarchy. Consequently,
> configured tiers may differ from effective ones, as they must be a subset
> of the parent's.
> 
> Note that cgroups do not pin swap tiers. This is similar to the
> `cpuset` controller, which does not prevent CPU hotplug. This
> approach ensures flexibility by allowing tier configuration changes
> regardless of cgroup usage.
> 
> Signed-off-by: Youngjun Park <youngjun.park@....com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 27 ++++++++
>  include/linux/memcontrol.h              |  3 +-
>  mm/memcontrol.c                         | 85 +++++++++++++++++++++++
>  mm/swap_state.c                         |  6 +-
>  mm/swap_tier.c                          | 89 ++++++++++++++++++++++++-
>  mm/swap_tier.h                          | 39 ++++++++++-
>  mm/swapfile.c                           |  4 ++
>  7 files changed, 246 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 7f5b59d95fce..776a908ce1b9 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1848,6 +1848,33 @@ 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
> +        A read-write nested-keyed file which exists on non-root

"nested-keyed" format is something else in this document's lingo, see
e.g. io.stat.

I think you wanted to make this resemble cgroup.subtree_control (which
is fine).

> +        cgroups. The default is to enable all tiers.
> +
> +        This interface allows selecting which swap tiers a cgroup can
> +        use for swapping out memory.
> +
> +        The effective tiers are inherited from the parent. Only tiers
> +        effective in the parent can be effective in the child. However,
> +        the child can explicitly disable tiers allowed by the parent.
> +
> +        When read, the file shows two lines:
> +          - The first line shows the operation string that was
> +            written to this file.
> +          - The second line shows the effective operation after
> +            merging with parent settings.

The convention (in cpuset) is to split it in two files like
memory.swap.tiers and memory.swap.tiers.effective.

> +
> +        When writing, the format is:
> +          (+/-)(TIER_NAME) (+/-)(TIER_NAME) ...
> +
> +        Valid tier names are those configured in
> +        /sys/kernel/mm/swap/tiers.
> +
> +        Each tier can be prefixed with:
> +          +    Enable this tier
> +          -    Disable this tier
> +

I believe these are only superficial adjustments not affecting the
implementation.

Thanks,
Michal

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