Mention explicitly how WSL 2 distros run as containers under a single kernel

[cerebrate]

This particular detail of the WSL architecture makes it behave in ways different from a "generic" Linux installation (e.g., kernel threads do not show up in a process listing, et. sim.), and these ways are unexpected if you don't know what's going on under the hood. And hence it would be useful to have that in the docs.

(Initially reported at microsoft/WSL#4622 .)

[craigloewen-msft]

Agreed! I aim to add a 'How WSL2' works doc as we approach general release and will make sure this detail is added in there.

[mattwojo]

@craigloewen-msft -- is this adequetly covered here? https://docs.microsoft.com/en-us/windows/wsl/compare-versions

If so, please close. :) ~Matt

[craigloewen-msft]

Not completely. Really the answer here would be creating a full 'How does WSL 2' work doc that basically explains this diagram:
https://www.youtube.com/watch?v=lwhMThePdIo&t=1800s

[mattwojo]

Sounds good. Drafted a WSL 2 architecture explainer doc -- could you help me fill it in with the correct info?
https://github.com/MicrosoftDocs/WSL/blob/mattw-wsl2-explainer/WSL/wsl2-architecture.md

[cerebrate]

It would probably be useful to include a quick reference list of things that aren't covered by the namespacing that separates distros.

If I'm reading lsns right, the distros have their own cgroup/user/net/uts/ipc/mnt/pid namespaces, but not time namespaces (understandably, since those have only been around since kernel 5.6 and the WSL kernel hasn't reached that yet), so the system clocks mentioned in time_namespaces(7) are shared between distros. Admittedly, these are nonsettable clocks outside any clock offsets you create while creating the namespace, so I can't think of a reason offhand why you'd want to do that for WSL distros anyway.

On the other hand, RTC time is shared between distros (with a potential relevant use case for changing time to run tests). Other things that come to mind are loaded kernel modules, the kernel keyring, kernel security policies, and probably a variety of other things I don't know Linux well enough to know of...

[vbrozik]

Not completely. Really the answer here would be creating a full 'How does WSL 2' work doc that basically explains this diagram: https://www.youtube.com/watch?v=lwhMThePdIo&t=1800s

The particular part about the WSL2 instances (Microsoft calls them "distributions") sharing a single VM, single Linux kernel instance and using namespaces is mentioned here:
https://youtu.be/lwhMThePdIo?t=2436

It would be really useful to have at least a rough description of these:

• how is the kernel booted
• how are the kernel parameters set
• how is the initial filesystem mounted
• how does the WSL's init work
• how are the namespaces created
• how are the filesystems of "distributions" mounted
• how are the "distributions" executed (do they use the same init binary?)