NAK/Cmnt: [SRU][N hwe/6.14][PATCH] Re-enable memcg v1 on Noble (6.14)

[Krister Johansen]

On Mon, Sep 08, 2025 at 03:25:04PM -0700, Krister Johansen wrote:
> BugLink: https://bugs.launchpad.net/bugs/2122368
> 
> [Impact]
> 
> Although v1 cgroups are deprecated in Noble, it was still possible for
> users on 6.8 kernels to utilize them.  This was especially helpful in
> helping migrating users to Noble and then separately upgrading their
> remaining v1 cgroups applications.  Instead of requiring all users to
> upgrade and fix their v2 support, v1 support could be provisionally
> enabled until the necessary support was available in the applications
> that still lack v2 support.
> 
> Starting in 6.12, CONFIG_MEMCG_V1 was added and defaulted to false.
> Noble 6.8 users that were unlucky enough to still need V1 cgroups found
> that they could no longer use memcgs in the 6.14 kernel.
> 
> Specific use cases include older JVMs that fail to correctly handle
> missing controllers from /proc/cgroups.  In that case, the container
> limit detection is turned off and the JVM uses the host's limits.
> 
> Further, Apache Yarn is still completing their v1 -> v2 migration, which
> leaves some Hadoop use cases without proper support.
> 
> The request here is to enable MEMCG_V1 on Noble, but not newer releases,
> for as long as the Noble HWE kernel train still has kernels with cgroup
> v1 support.  This gives users a little bit longer to complete their
> migration while still using newer hardware, but with the understanding
> that this really is the end of the line for v1 cgroups.
> 
> [Fix]
> 
> Re-enable CONFIG_MEMCG_V1 in the 6.14 Noble config.

I got some feedback about this patch off-list and will spin a v2 that
incorporates the requested changes.

-K