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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Mar 19, 2025 at 03:38:38PM -0400, Johannes Weiner wrote:
> On Wed, Mar 19, 2025 at 02:41:43PM +0800, Jingxiang Zeng wrote:
> > From: Zeng Jingxiang <linuszeng@tencent.com>
> > 
> > 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.

At the moment memsw and swap shares the underlying page_counter. This
would require having explicit page_counter for memsw.

What's your take on memsw interfaces still behind
CONFIG_MEMSW_ACCOUNT_ON_DFL?