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List:       openjdk-hotspot-runtime-dev
Subject:    Re: SIGBUS on linux in perfMemory_init (containers)
From:       Ioi Lam <ioi.lam () oracle ! com>
Date:       2022-04-29 21:19:00
Message-ID: f15471cd-4915-5e08-955f-b6d094f96b14 () oracle ! com
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On 4/29/2022 1:55 PM, Ioi Lam wrote:
> 
> 
> On 4/29/2022 6:44 AM, Vitaly Davidovich wrote:
> > Hi all,
> > 
> > We've been seeing intermittent SIGBUS failures on linux with jdk11.  
> > They
> > all have this distinctive backtrace:
> > 
> > C  [libc.so.6+0x12944d]
> > 
> > V  [libjvm.so+0xcca542]  perfMemory_init()+0x72
> > 
> > V  [libjvm.so+0x8a3242]  vm_init_globals()+0x22
> > 
> > V  [libjvm.so+0xedc31d]  Threads::create_vm(JavaVMInitArgs*, 
> > bool*)+0x1ed
> > 
> > V  [libjvm.so+0x9615b2]  JNI_CreateJavaVM+0x52
> > 
> > C  [libjli.so+0x49af]  JavaMain+0x8f
> > 
> > C  [libjli.so+0x9149]  ThreadJavaMain+0x9
> > 
> > 
> > Initially, we suspected that /tmp was full but that turned out to not be
> > the case.  After a few more instances of the crash and investigation, we
> > believe we know the root cause.
> > 
> > 
> > The crashing applications are all running in a K8 pod, with each JVM 
> > in a
> > separate container:
> > 
> > 
> > container_type: cgroupv1 (from the hs_err file)
> > 
> > 
> > /tmp is mounted such that it's shared by multiple containers. Since 
> > these
> > JVMs are running in containers, we believe what happens is the 
> > namespaced
> > (i.e. per container) PIDs overlap between different containers - 2 
> > JVMs, in
> > separate containers, can end up with the same namespaced PID. Since /tmp
> > is shared, they can now "contend" on the same perfMemory file since 
> > those
> > file names are PID based.
> 
> Hi Vitaly,
> 
> Is there any reason for sharing the same /tmp directory across 
> different containers?
> 
> Are you using the /tmp/hsperfdata_$USER/<pid> files at all. If not, 
> for the time being, you can disable them with the -XX:-UsePerfData flag,
> 
> https://bugs.openjdk.java.net/browse/JDK-8255008 has a related proposal:
> 
> Java: -Djdk.attach.tmpdir=/container-attachdir 
> -XX:+UnlockCommercialFeature -XX:+FlightRecorder -XX:+StartAttachListener
> Docker: --volume /tmp/container-attachdir:/container-attachdir
> 
> In this case, we probably will run into the same UID clash problem as 
> well.
> 
> Maybe we should have an additional property like 
> -Djdk.attach.use.global.pid=true
> 

I read the proposal in JDK-8255008 again and realized that the JVM 
inside the container doesn't know what it's host PID is. The proposal is 
to create these files:

$jdk_attach_dir/hsperfdata_{user}/e4f3e2e4fd97:10
$jdk_attach_dir/.java_pid:e4f3e2e4fd97:10

where the e4f3e2e4fd97 is the container ID which is visible as 
/tmp/hostname from inside the container.

I'll try to implement a prototype for the proposal.

Thanks
- Ioi

> 
> > 
> > Once multiple JVMs can contend on the same file, a SIGBUS can arise 
> > if one
> > JVM has mmap'd the file and another ftruncate()'s it from under it (e.g.
> > https://github.com/openjdk/jdk11/blob/37115c8ea4aff13a8148ee2b8832b20888a5d880/src/hotspot/os/linux/perfMemory_linux.cpp#L909 \
> >  
> > ).
> > 
> > 
> > Is this a known issue? I couldn't find any existing JBS entries or 
> > mailing
> > list discussions around this specific circumstance.
> > 
> > 
> > As for possible solutions, would it be possible to use the global PID
> > instead of the namespaced PID to "regain" the uniqueness invariant of 
> > the
> > PID?
> 
> I think this needs to be optional. E.g., if you run a tool such as 
> "jcmd" inside the same container as the jvm process, the jcmd tool 
> would expect the PID to be the local version specific to this container.
> 
> > Also, might it make sense to flock() the file to prevent another
> > process from mucking with it?
> > 
> That will not solve the fundamental problem where two processes are 
> using the same hsperfdata file. One of them would fail to write to it, 
> and tools won't be able to monitor both processes.
> 
> Thanks
> - Ioi
> 
> > Happy to provide more info if needed.
> > 
> > 
> > Thanks
> 


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