On Thu, 7 Jul 2022 21:21:27 GMT, Ioi Lam <[email protected]> wrote:
>> Some Kubernetes setups share the /tmp directory across multiple containers.
>> On rare occasions, the JVM may crash when it tries to write to
>> `/tmp/hsperfdata_<user>/<pid>` when a process in a separate container
>> decides to do the same thing (because they happen to have the same
>> namespaced pid).
>>
>> This patch avoids the crash by using `flock()` to allow only one of these
>> processes to write to the file. All other competing processes that fail to
>> grab the lock will give up the file and run with PerfMemory disabled. We
>> will try to enable PerfMemory for the failed processes in a follow-up RFE:
>> [JDK-8289883](https://bugs.openjdk.org/browse/JDK-8289883)
>>
>> Thanks to Vitaly Davidovich and Nico Williams for coming up with the idea of
>> using `flock()`.
>>
>> I kept the use of `kill()` for stale file detection to be compatible with
>> older JVMs.
>>
>> I also took the opportunity to clean up the comments and remove dead code.
>> The old code was using "shared memory resources" which sounds unclear and
>> odd. I changed the terminology to say "shared memory file" instead.
>
> Ioi Lam has updated the pull request incrementally with one additional commit
> since the last revision:
>
> @tstuefe comments
src/hotspot/os/posix/perfMemory_posix.cpp line 743:
> 741:
> 742: // We now have a file name that converts to a valid integer
> 743: // that could represent a process id.
"could"? It does, or? Why being ambiguous? (If we change the scheme later, we
can be more specific)
src/hotspot/os/posix/perfMemory_posix.cpp line 744:
> 742: // We now have a file name that converts to a valid integer
> 743: // that could represent a process id.
> 744: //
Can you add a short introduction here similar to:
"On Linux, VMs may run in different PID namespaces and therefore their PIDs are
not unique. Therefore, we first try to flock() the file ..."
src/hotspot/os/posix/perfMemory_posix.cpp line 759:
> 757: // signal the process, then the file is assumed to
> 758: // be stale and is removed because the files for such a
> 759: // process should be in a different user specific directory.
Please add a short sentence like:
"Finally, if VMs from different containers share /tmp, we cannot reliably
determine process liveness using kill. To avoid that, either stop sharing /tmp
between containers or only run VMs with the JDK-8286030 fix."
src/hotspot/os/posix/perfMemory_posix.cpp line 766:
> 764: // The locking protocol works only with JVMs that have the
> JDK-8286030 fix.
> 765: // If you are sharing the /tmp difrectory among different
> containers, do not
> 766: // use older JVMs that don't have this fix.
Could possible shorten this if you feel the long comment above explains it all
already.
src/hotspot/os/posix/perfMemory_posix.cpp line 796:
> 794: if (!is_locked_by_another_process) {
> 795: if ((pid == os::current_process_id()) ||
> 796: (kill(pid, 0) == OS_ERR && (errno == ESRCH || errno ==
> EPERM))) {
Thinking about this, I now was confused. AFAICS this code is only ever called
from `mmap_create_shared` with the directory containing our own pid. I do not
see this called for PIDs from other processes. Why do we handle that case? Or
am I overlooking something?
-------------
PR: https://git.openjdk.org/jdk/pull/9406
