On Linux, (how) can I "hot-unplug" memory?

Question

This is what I've tried:

sudo zsh -c 'echo offline >/sys/devices/system/memory/memory109/state'`

and got the following error:

zsh:echo:1: write error: device or resource busy

Do I perhaps need to make sure that the memory module that I am about to take offline is empty first?

And if so, how could I do that?

---

Background:

I'm running an Ubuntu based distro virtualized as a guest by VMWare.

I'm hoping to reduce the memory usage of the guest system, freeing up more to be available to the host without having to restart the guest.

In case any of that may matter.

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Edit 2016-09-29:

Encouraged by @Gilles' comments and link saying that in theory it should be possible, I got the idea that maybe memory109 contained some unmovable pages. So I decided to try it on some other modules(?) too.

At first I tried a few others by hand. But since those were unsuccessful, I thought I would try all of them:

for i in /sys/devices/system/memory/memory*/state;
  do echo -n "Trying "$i" \t";
  sudo zsh -c "echo offline > $i";
done

Caution: Be careful with the above script as it may succeed on your machine which could offline most if not all of your memory leaving your system starved and hanging.

This is what I got for most:

...
Trying /sys/devices/system/memory/memory44/state        zsh:echo:1: write error: device or resource busy
Trying /sys/devices/system/memory/memory45/state        zsh:echo:1: write error: device or resource busy
Trying /sys/devices/system/memory/memory46/state        zsh:echo:1: write error: device or resource busy
...

With only two exceptions:

Trying /sys/devices/system/memory/memory0/state         zsh:echo:1: write error: invalid argument
...
Trying /sys/devices/system/memory/memory86/state        zsh:echo:1: write error: invalid argument
...

Any ideas why this may not work?

Answer 1

Most likely, your memory blocks cannot be offlined, because they contain unmovable pages. Check the "removable" sysfs attribute.

If you're runing a normal x86-64 system (virtualized or bare metal), it's very likely that there are no removable blocks, because all kernel allocations are unmovable. To restrict the kernel to some portion of ram, pass a "kernelcore=" parameter to it. You should see some removable memory blocks after that, and you can offline them either by writing '0' to the 'online' attribute or by writing 'offline' to the 'state' attribute.

Answer 2

I don't think it's possible by kernel's default features. Kernel only supports hot-plug, but unplug. There could be a weirdo way to do this, but technically it sounds bad... there are few problems with this at least.