util: fix physical_memory() to work correctly on cgroupsv2

Let's look into the right cgroupsv2 attribute.

Also, while we are at it, add debug logging for all error conditions we
eat up silently otherwise.

src/basic/util.c

@@ -405,6 +405,7 @@ uint64_t physical_memory(void) {
         uint64_t mem, lim;
         size_t ps;
         long sc;
+        int r;
 
         /* We return this as uint64_t in case we are running as 32bit process on a 64bit kernel with huge amounts of
          * memory.
@@ -418,13 +419,40 @@ uint64_t physical_memory(void) {
         ps = page_size();
         mem = (uint64_t) sc * (uint64_t) ps;
 
-        if (cg_get_root_path(&root) < 0)
+        r = cg_get_root_path(&root);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to determine root cgroup, ignoring cgroup memory limit: %m");
                 return mem;
+        }
 
-        if (cg_get_attribute("memory", root, "memory.limit_in_bytes", &value))
+        r = cg_all_unified();
+        if (r < 0) {
+                log_debug_errno(r, "Failed to determine root unified mode, ignoring cgroup memory limit: %m");
                 return mem;
+        }
+        if (r > 0) {
+                r = cg_get_attribute("memory", root, "memory.max", &value);
+                if (r < 0) {
+                        log_debug_errno(r, "Failed to read memory.max cgroup attribute, ignoring cgroup memory limit: %m");
+                        return mem;
+                }
 
-        if (safe_atou64(value, &lim) < 0)
+                if (streq(value, "max"))
+                        return mem;
+        } else {
+                r = cg_get_attribute("memory", root, "memory.limit_in_bytes", &value);
+                if (r < 0) {
+                        log_debug_errno(r, "Failed to read memory.limit_in_bytes cgroup attribute, ignoring cgroup memory limit: %m");
+                        return mem;
+                }
+        }
+
+        r = safe_atou64(value, &lim);
+        if (r < 0) {
+                log_debug_errno(r, "Failed to parse cgroup memory limit '%s', ignoring: %m", value);
+                return mem;
+        }
+        if (lim == UINT64_MAX)
                 return mem;
 
         /* Make sure the limit is a multiple of our own page size */