README: say kernel 4.15 is the minimum recommended

After various long discussions
(https://lists.freedesktop.org/archives/systemd-devel/2022-March/047587.html,
https://lwn.net/Articles/889610/), there is no clear answer what the minimum
version should be. Bumping the version above 3.15 doesn't allow us to make any
significant simplifications (unless we went *much* higher). In particular, even
renameat2() is not fully supported with latest kernel versions, e.g. nfs still
doesn't have it. And the bpf stuff is optional anyway. So let's just say that
4.15 is what we recommend, because it provides fairly complete cgroups-v2, but
without any removals of compat in the code.

diff --git a/README b/README
index 420276eda8951ce6e323559d0a0a6562e025f451..06c54e04d735425d4da3d1f424382e44581a3bce 100644 (file)
--- a/README
+++ b/README
@@ -30,14 +30,21 @@ LICENSE:
         LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md
 
 REQUIREMENTS:
-        Linux kernel >= 3.15
-        Linux kernel >= 4.2 for unified cgroup hierarchy support
-        Linux kernel >= 4.10 for cgroup-bpf egress and ingress hooks
-        Linux kernel >= 4.15 for cgroup-bpf device hook
-        Linux kernel >= 4.17 for cgroup-bpf socket address hooks
-        Linux kernel >= 5.3 for bounded-loops in BPF program
-        Linux kernel >= 5.4 for signed Verity images support
-        Linux kernel >= 5.7 for BPF links and the BPF LSM hook
+        Linux kernel ≥ 3.15
+                     ≥ 4.5 for pids controller in cgroup v2
+                     ≥ 4.6 for cgroup namespaces
+                     ≥ 4.9 for RENAME_NOREPLACE support in vfat
+                     ≥ 4.10 for cgroup-bpf egress and ingress hooks
+                     ≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
+                     ≥ 4.17 for cgroup-bpf socket address hooks
+                     ≥ 5.3 for bounded loops in BPF program
+                     ≥ 5.4 for signed Verity images
+                     ≥ 5.7 for BPF links and the BPF LSM hook
+
+        Kernel versions below 4.15 have significant gaps in functionality and
+        are not recommended for use with this version of systemd. Taint flag
+        'old-kernel' will be set. Systemd will most likely still function, but
+        upstream support and testing are limited.
 
         Kernel Config Options:
           CONFIG_DEVTMPFS

diff --git a/src/basic/def.h b/src/basic/def.h
index 54a82c7c49a2501df438f5cf49df2797aa863334..0a1ae023a37cf263556e3a879cf760b5e8ae783c 100644 (file)
--- a/src/basic/def.h
+++ b/src/basic/def.h
@@ -74,4 +74,4 @@
 /* Path where systemd-oomd listens for varlink connections from user managers to report changes in ManagedOOM settings. */
 #define VARLINK_ADDR_PATH_MANAGED_OOM_USER "/run/systemd/oom/io.system.ManagedOOM"
 
-#define KERNEL_BASELINE_VERSION "3.15"
+#define KERNEL_BASELINE_VERSION "4.15"