Re: [RFC 0/5] add option to restore swap account to cgroupv1 mode

[Johannes Weiner]

On Wed, Mar 19, 2025 at 02:41:43PM +0800, Jingxiang Zeng wrote:
> From: Zeng Jingxiang <linuszeng@xxxxxxxxxxx>
> 
> memsw account is a very useful knob for container memory
> overcommitting: It's a great abstraction of the "expected total
> memory usage" of a container, so containers can't allocate too
> much memory using SWAP, but still be able to SWAP out.
> 
> For a simple example, with memsw.limit == memory.limit, containers
> can't exceed their original memory limit, even with SWAP enabled, they
> get OOM killed as how they used to, but the host is now able to
> offload cold pages.
> 
> Similar ability seems absent with V2: With memory.swap.max == 0, the
> host can't use SWAP to reclaim container memory at all. But with a
> value larger than that, containers are able to overuse memory, causing
> delayed OOM kill, thrashing, CPU/Memory usage ratio could be heavily
> out of balance, especially with compress SWAP backends.
> 
> This patch set adds two interfaces to control the behavior of the
> memory.swap.max/current in cgroupv2:
> 
> CONFIG_MEMSW_ACCOUNT_ON_DFL
> cgroup.memsw_account_on_dfl={0, 1}
> 
> When one of the interfaces is enabled: memory.swap.current and
> memory.swap.max represents the usage/limit of swap.
> When neither is enabled (default behavior),memory.swap.current and
> memory.swap.max represents the usage/limit of memory+swap.

This should be new knobs, e.g. memory.memsw.current, memory.memsw.max.

Overloading the existing swap knobs is confusing.

And there doesn't seem to be a good reason to make the behavior
either-or anyway. If memory.swap.max=max (default), it won't interfere
with the memsw operation. And it's at least conceivable somebody might
want to set both, memsw.max > swap.max, to get some flexibility while
excluding the craziest edge cases.