RFR: 8321931: memory_swap_current_in_bytes reports 0 as "unlimited"

[Ioi Lam]

On Tue, 16 Jan 2024 22:36:48 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

> > I'll have a look tomorrow. In general unlimited means `-1` not `0`. cg v1 doesn't explicitly set it to a specified value if it is unlimited. It's a very large number. For cg v2 it's usually the string `max` as specified by the kernel docs of interface files.
> 
> Thank you for taking a look. 

According to
https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git/tree/Documentation/admin-guide/cgroup-v2.rst


 memory.swap.max
	A read-write single value file which exists on non-root
	cgroups.  The default is "max".

	Swap usage hard limit.  If a cgroup's swap usage reaches this
	limit, anonymous memory of the cgroup will not be swapped out.


E.g.,


$ cat /sys/fs/cgroup/user.slice/memory.swap.max
max


I don't think there's a file under /sys/fs/cgroup where the numberic value of "0" means "unlimited". Instead, "0" means zero or "not allowed at all".

Otherwise, how can you express "don't allow any swapping for this container"?

I think the proper fix is:


void OSContainer::print_container_helper(outputStream* st, jlong j, const char* metrics) {
  st->print("%s: ", metrics);
- if (j > 0) {
+ if (j >= 0) {


> I think I saw this special treatment of 0 value somewhere in source code. I will have to look.

It's worthwhile scanning the source code to for such cases and possibly fixing them.

In most cases, I see that -1 means unlimited, and -2 (OSCONTAINER_ERROR) means error: 


jlong CgroupSubsystem::limit_from_str(char* limit_str) {
  if (limit_str == nullptr) {
    return OSCONTAINER_ERROR;
  }
  // Unlimited memory in cgroups is the literal string 'max' for
  // some controllers, for example the pids controller.
  if (strcmp("max", limit_str) == 0) {
    os::free(limit_str);
    return (jlong)-1;
  }
  julong limit;
  if (sscanf(limit_str, JULONG_FORMAT, &limit) != 1) {
    os::free(limit_str);
    return OSCONTAINER_ERROR;
  }
  os::free(limit_str);
  return (jlong)limit;
}

-------------

PR Comment: https://git.openjdk.org/jdk/pull/17314#issuecomment-1895272329