sys-net has too little memory if 4K screen is connected

[marmarek]

Qubes OS release

Qubes OS 4.3

Brief summary

Current R4.3 default for sys-net is 300MB. If 4K screen is connected (or simply multiple screens with total resolution about this size), the framebuffer for the root window takes considerable amount of memory, specifically about 32MB. In light of 300MB total, it's not negligible. And results in OOM-killer killing Xorg at startup.

Steps to reproduce

1. Connect multiple displays, or one with 4K resolution
2. Start sys-net

Expected behavior

Starts normally

Actual behavior

Observe Xorg killed in sys-net. No network manager icon or any window from sys-net works.

Additional information

Interestingly, sys-usb works fine. It's likely related to no NetworkManager there, which is another process taking considerable amount of memory.

[marmarek]

TBH, it feels kinda pointless to need that memory. Root window in the VM isn't really used for anything, as there is no display attached. Maybe for screenshots/screensharing, but that isn't really needed in sys-net (or sys-usb) either.
But looking at the driver code, I don't see an easy way to avoid it, looks like Xorg does expect it to be there. See the fbScreenInit call and parts after that.

Hopefully Wayland won't need that in the VM...

[marmarek]

In the meantime, it looks like we need to increase memory for sys-net (again) if higher resolution display(s) are in use.

[marmarek]

Maybe the root window can be excluded from allocating grant refs for it? It isn't mapped to GUI domain ever, so it doesn't need them. That should allow swapping it if necessary (memory allocated for sharing is basically mlock-ed into actual RAM).