improve error message for cgroup limit

[medyagh]

❗ Cgroup v2 does not allow setting memory, if you want to set memory, please modify your Grub as instructed in https://docs.docker.com/engine/install/linux-postinstall/#your-k
ernel-does-not-support-cgroup-swap-limit-capabilities

[tstromberg]

I was just going to open an issue relating to this. This warning should probably only show up if someone is actually setting a memory restriction. 99% of folks won't care otherwise.

[afbjorklund]

Why does it say "Cgroup v2" ? And what was wrong with the original message, as issued by Docker ? (for v1)

WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted.

[afbjorklund]

Was added in #10512

But I think it should actually have been --memory-swap -1 to limit just the memory and not the swap.

From #10371 (comment)

Reopened that issue, it was closed by the bot by an earlier commit (when I thought it was just a warning)

[hetong07]

Sorry that I misunderstood this issue #10371 , so I added a wrong warning message. I saw different materials, and one said it is the cgroup v2 problem and the other said the error in #10371 is because it cannot set memory limit. So somehow I linked them together and wrote this inaccurate warning message.

[afbjorklund]

I think the confusing part is that Docker handles cgroup v1 and v2 very differently, with regards to the parameters.

Then again, WARNING: Support for cgroup v2 is experimental

[medyagh]

@hetong07 I agree with @tstromberg if user did not Set --memory by flag ( we should NOT warn the user)
and also lets make the warnning something more elegant how about making it to Two lines

❗ Your cgroup does not allow setting memory.
📘  More information: https://docs.docker.com/engine/install/linux-postinstall/#your-kernel-does-not-support-cgroup-swap-limit-capabilities

[hetong07]

/assign