NixOS: increase default value of vm.max_map_count

[SuperSamus]

Describe the bug

Applications are starting to allocate more times than allowed by the default value of vm.max_map_count, 65530.

For instance, this situation arises in applications such as Ryujinx (Ryujinx/Ryujinx#4702) and newer Windows games like Hogwarts Legacy (HansKristian-Work/vkd3d-proton#1420).

SteamOS has already addressed this matter by augmenting the aforementioned value, while Fedora intends to implement the same modification in Fedora 39 (https://www.phoronix.com/news/Fedora-39-Max-Map-Count-Approve, https://fedoraproject.org/wiki/Changes/IncreaseVmMaxMapCount).

[RaitoBezarius]

Please feel free to send PRs, it's hard to track those issues as you don't mention any owner.

[SuperSamus]

The line to add is:

boot.kernel.sysctl."vm.max_map_count" = mkDefault n;

The questions are:

• How big should n be? 2147483642? 1048576?
• In which file should I add it? At the bottom of nixos/modules/config/sysctl.nix?

[RaitoBezarius]

The line to add is:

boot.kernel.sysctl."vm.max_map_count" = mkDefault n;

The questions are:

• How big should n be? 2147483642? 1048576?
• In which file should I add it? At the bottom of nixos/modules/config/sysctl.nix?

You mentioned other distros were thinking about bumping it, let's follow Fedora lead

In the relevant file, a quick grep shows indeed that sysctl is one of the best candidate. :)

[yu-re-ka]

For the policy discussion: I'm all in favor of doing this soon(m). It is not really dangerous and as people have explained to me the previous default has been there for historical reasons and 32 bit systems.