Getting memory usage fails from missing information on WSL

[CosmicHorrorDev]

Please provide any of the following information if relevant:

• rust-psutil version: 3.1.0 (issue is still present in master)
• target platform info
  • kernel version: uname -a says 4.4, but behavior seems to deviate from mainline kernel
  • architecture: x86_64
  • (if linux) distro: WSL ubuntu 18.04
• any relevant hardware info:

From cjbassi/ytop#106

In ytop, users trying to run the program with WSL were encountering issues where calling pstuil::memory::virtual_memory() will fail with MissingData due to /proc/meminfo missing MemAvailable. From my brief reading on the topic, this was an attribute that was added in kernel 3.4 (hence my confusion on version 4.4) to give an accurate idea on the amount of available memory left on the system. You can read more on the discussion here where it also mentions that you:

can be estimated from MemFree, Active(file), Inactive(file), and SReclaimable, as well as the "low" watermarks from /proc/zoneinfo.

From this it might be reasonable to provide a fallback where the value is estimated if MemAvailable isn't found in /proc/meminfo.

[cjbassi]

The reason the field is missing is because WSL uses a custom kernel build with that field disabled.

Here's how python psutil handles things: https://github.com/giampaolo/psutil/blob/6242f7411b882d525e5d267de4bcda1079934ea2/psutil/_pslinux.py#L321