linux: enable kcmp() system call

[blitz]

Motivation for this change

Since 2020, Mesa requires the kcmp() system call to be available for some of its functionality. At the moment, this system call is enabled when CHECKPOINT_RESTORE is enabled in the Linux kernel. This option is also enabled by default in Fedora and Debian.

This PR enables CHECKPOINT_RESTORE by default on all kernel versions where it is not marked as EXPERT, i.e. everything after and including 4.19.

Fixes #112082.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • kernel-lts
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[blitz]

I've verified that everything <=4.14 has no config changes. The config changes for the Linux >=4.19 default config look like this:

--- config-4.19-old	2021-02-15 23:01:28.626391734 +0100
+++ result	1970-01-01 01:00:01.000000000 +0100
@@ -155,7 +155,7 @@
 CONFIG_USER_NS=y
 CONFIG_PID_NS=y
 CONFIG_NET_NS=y
-# CONFIG_CHECKPOINT_RESTORE is not set
+CONFIG_CHECKPOINT_RESTORE=y
 CONFIG_SCHED_AUTOGROUP=y
 # CONFIG_SYSFS_DEPRECATED is not set
 CONFIG_RELAY=y
@@ -998,6 +998,7 @@
 CONFIG_CLEANCACHE=y
 CONFIG_FRONTSWAP=y
 # CONFIG_CMA is not set
+# CONFIG_MEM_SOFT_DIRTY is not set
 CONFIG_ZSWAP=y
 CONFIG_ZPOOL=y
 CONFIG_ZBUD=y
@@ -8095,7 +8096,7 @@
 CONFIG_PROC_KCORE=y
 CONFIG_PROC_SYSCTL=y
 CONFIG_PROC_PAGE_MONITOR=y
-# CONFIG_PROC_CHILDREN is not set
+CONFIG_PROC_CHILDREN=y
 CONFIG_KERNFS=y
 CONFIG_SYSFS=y
 CONFIG_TMPFS=y

[vcunat]

As written in this PR, features.criu doesn't change anything on newer kernels and that seems wrong (by looking at the diff; I don't know anything about criu really).

[blitz]

As written in this PR, features.criu doesn't change anything on newer kernels and that seems wrong (by looking at the diff; I don't know anything about criu really).

features.criu doesn't do anything on newer kernels because checkpoint/restore is always available. The diff looks ok from my end, what issue you do you see?

I refrained from enabling checkpoint/restore by default on older kernels, because there it probably was hidden on purpose behind EXPERT and might have issues. People who ask for it using programs.criu.enable still get the support either way.

[vcunat]

I said it wrong. I meant features.criu_revert_expert. It did act on older kernels before this PR and that property is gone by this commit without explanation.

[blitz]

I said it wrong. I meant features.criu_revert_expert. It did act on older kernels before this PR and that property is gone by this commit without explanation.

This should work exactly as before unless I set the parentheses wrong.

[primeos]

I didn't test this but the diff LGTM :)

The current solution by @blitz looks quite elegant to me:

    criu = if (versionAtLeast version "4.19") then {
      # Unconditionally enabled, because it is required for CRIU and
      # it provides the kcmp() system call that Mesa depends on.
      CHECKPOINT_RESTORE  = yes;
    } else (optionalAttrs (features.criu or false) {
      # For older kernels, CHECKPOINT_RESTORE is hidden behind EXPERT.
      EXPERT              = yes;
      CHECKPOINT_RESTORE  = yes;
    } // optionalAttrs (features.criu_revert_expert or true) {
      RFKILL_INPUT          = option yes;
      HID_PICOLCD_FB        = option yes;
      HID_PICOLCD_BACKLIGHT = option yes;
      HID_PICOLCD_LCD       = option yes;
      HID_PICOLCD_LEDS      = option yes;
      HID_PICOLCD_CIR       = option yes;
      DEBUG_MEMORY_INIT     = option yes;
    });

So basically for Linux kernels >= 4.19 we unconditionally set CHECKPOINT_RESTORE and for older kernels the behaviour should be unchanged.
(Edit: I think I got it now, I'll have another look.)

[primeos]

I was about to merge this and finally noticed what @vcunat meant (at least I guess that's it - nice catch btw!):

--- a/pkgs/os-specific/linux/kernel/common-config.nix
+++ b/pkgs/os-specific/linux/kernel/common-config.nix
@@ -638,7 +638,7 @@ let
       # Unconditionally enabled, because it is required for CRIU and
       # it provides the kcmp() system call that Mesa depends on.
       CHECKPOINT_RESTORE  = yes;
-    } else (optionalAttrs (features.criu or false) {
+    } else optionalAttrs (features.criu or false) ({
       # For older kernels, CHECKPOINT_RESTORE is hidden behind EXPERT.
       EXPERT              = yes;
       CHECKPOINT_RESTORE  = yes;

That way the (features.criu_revert_expert or true) part, that is added by default, will only be added if (features.criu or false) (which was the old behaviour).

I've verified that everything <=4.14 has no config changes.

I can confirm this. I didn't investigate this further but I assume the (features.criu_revert_expert or true) part matches the Linux kernel defaults (I only tested it on 4.14 though). Anyway, now we should be on the safe side :)

I also switched this PR from master to staging because most of the other PRs I've seen changing the Linux kernel configuration where merged into staging as well (though from the amount of rebuilds master should be fine AFAIK).

[primeos]

Let's give this a try :) Thanks a lot @blitz!