core: add cgroup memory controller support on the unified hierarchy (#3315)

On the unified hierarchy, memory controller implements three control knobs -
low, high and max which enables more useable and versatile control over memory
usage.  This patch implements support for the three control knobs.

* MemoryLow, MemoryHigh and MemoryMax are added for memory.low, memory.high and
  memory.max, respectively.

* As all absolute limits on the unified hierarchy use "max" for no limit, make
  memory limit parse functions accept "max" in addition to "infinity" and
  document "max" for the new knobs.

* Implement compatibility translation between MemoryMax and MemoryLimit.

v2:

- Fixed missing else's in config_parse_memory_limit().
- Fixed missing newline when writing out drop-ins.
- Coding style updates to use "val > 0" instead of "val".
- Minor updates to documentation.

1 files changed, 15 insertions, 10 deletions

diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 86b4fb071b..09d3f65c77 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -2793,21 +2793,26 @@ int config_parse_memory_limit(
                 void *userdata) {
 
         CGroupContext *c = data;
-        uint64_t bytes;
+        uint64_t bytes = CGROUP_LIMIT_MAX;
         int r;
 
-        if (isempty(rvalue) || streq(rvalue, "infinity")) {
-                c->memory_limit = (uint64_t) -1;
-                return 0;
+        if (!isempty(rvalue) && !streq(rvalue, "infinity") && !streq(rvalue, "max")) {
+                r = parse_size(rvalue, 1024, &bytes);
+                if (r < 0 || bytes < 1) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Memory limit '%s' invalid. Ignoring.", rvalue);
+                        return 0;
+                }
         }
 
-        r = parse_size(rvalue, 1024, &bytes);
-        if (r < 0 || bytes < 1) {
-                log_syntax(unit, LOG_ERR, filename, line, r, "Memory limit '%s' invalid. Ignoring.", rvalue);
-                return 0;
-        }
+        if (streq(lvalue, "MemoryLow"))
+                c->memory_low = bytes;
+        else if (streq(lvalue, "MemoryHigh"))
+                c->memory_high = bytes;
+        else if (streq(lvalue, "MemoryMax"))
+                c->memory_max = bytes;
+        else
+                c->memory_limit = bytes;
 
-        c->memory_limit = bytes;
         return 0;
 }