linux-kernel - Re: [PATCH 1/4] mm/swap, memcg: Introduce infrastructure for cgroup-based swap priority

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Message-ID: <jrkh2jy2pkoxgsxgsstpmijyhbzzyige6ubltvmvwl6fwkp3s7@kzc24pj2tcko>
Date: Tue, 22 Jul 2025 10:41:20 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Youngjun Park <youngjun.park@....com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...nel.org, 
	roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, muchun.song@...ux.dev, 
	shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com, bhe@...hat.com, 
	baohua@...nel.org, chrisl@...nel.org, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, gunho.lee@....com, iamjoonsoo.kim@....com, taejoon.song@....com
Subject: Re: [PATCH 1/4] mm/swap, memcg: Introduce infrastructure for
 cgroup-based swap priority

On Thu, Jul 17, 2025 at 05:20:03AM +0900, Youngjun Park <youngjun.park@....com> wrote:
> +  memory.swap.priority
> +    A read-write flat-keyed file which exists on non-root cgroups.
> +    This interface allows you to set per-swap-device priorities for the current
> +    cgroup and to define how they differ from the global swap system.
> +
> +    To assign priorities or define specific behaviors for swap devices
> +    in the current cgroup, write one or more lines in the following
> +    formats:
> +
> +     - <swap_device_id> <priority>
> +     - <swap_device_id> disabled
> +     - <swap_device_id> none
> +     - default none
> +     - default disabled
> +
> +    Each <swap_device_id> refers to a unique swap device registered
> +    in the system. You can check the ID, device path, and current
> +    priority of active swap devices through the `/proc/swaps` file.

Do you mean row number as the ID? Or does this depend on some other
patches or API?


> +    This provides a clear mapping between swap devices and the IDs
> +    used in this interface.
> +
> +    The 'default' keyword sets the fallback priority behavior rule for
> +    this cgroup. If no specific entry matches a swap device, this default
> +    applies.
> +
> +    * 'default none': This is the default if no configuration
> +      is explicitly written. Swap devices follow the system-wide
> +      swap priorities.
> +
> +    * 'default disabled': All swap devices are excluded from this cgroup’s
> +      swap priority list and will not be used by this cgroup.

This duplicates memory.swap.max=0. I'm not sure it's thus necessary.
At the same time you don't accept 'default <priority>' (that's sane).


> +
> +    The priority semantics are consistent with the global swap system:
> +
> +      - Higher numerical values indicate higher preference.
> +      - See Documentation/admin-guide/mm/swap_numa.rst for details on
> +        swap NUMA autobinding and negative priority rules.
> +
> +    The handling of negative priorities in this cgroup interface
> +    has specific behaviors for assignment and restoration:
> +
> +    * Negative Priority Assignment

Even in Documentation/admin-guide/mm/swap_numa.rst it's part of "Implementation details".
I admit I'm daunted by this paragraphs. Is it important for this interface?


Thanks,
Michal

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