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

[YoungJun Park <youngjun.park@lge.com>]

On Tue, Feb 03, 2026 at 11:54:41AM +0100, Michal Koutný wrote:
> 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@lge.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@lge.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).

You are right, I used the wrong expression. 
Simply describing it as a "file" seems sufficient.

> 
> > +        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.

I will separate the two according to the convention. 
Thanks for correction.

> > +
> > +        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

Thanks for the review, Michal.
Youngjun Park