A Bug about the JVM Attach mechanism

[David Holmes]

Hi Peter,

On 21/06/2019 7:55 am, Peter Levart wrote:
> As far as I know, cron jobs that cleanup /tmp typically remove files 
> that have not been modified for a while.
> 
> On Fedora for example, there is a systemd timer that triggers once per 
> day and executes systemd-tmpfiles which manages volatile and temporary 
> files and directories. The configuration for /tmp is the following:
> 
> # Clear tmp directories separately, to make them easier to override
> q /tmp 1777 root root 10d
> q /var/tmp 1777 root root 30d
> 
> The age field (10 days for /tmp) has the following meaning:
> 
>         The age of a file system entry is determined from its last 
> modification timestamp (mtime), its last access timestamp (atime), and 
> (except for directories) its last status change
>         timestamp (ctime). Any of these three (or two) values will 
> prevent cleanup if it is more recent than the current time minus the age 
> field.
> 
> So the solution could be for attach thread (if it is already started) to 
> update mtime or ctime of the .java_pid<pid> socket file periodically so 
> cleanup job would leave it alone.
> 
> What do you think?

I'm not keen on having the attach listener thread periodically wakeup 
just to do this. Can we not change permissions to protect the file form 
external deletion and ensure the VM cleans it up itself?

David

> Regards, Peter
> 
> 
> On 6/20/19 10:49 PM, David Holmes wrote:
>> Sorry it took me a while to understand the specifics of the problem. :)
>>
>> David
>>
>> On 20/06/2019 3:37 am, nijiaben wrote:
>>> Yes Alan, I mean this
>>> ------------------ Original ------------------
>>> *From: * "Alan Bateman"<Alan.Bateman at oracle.com>;
>>> *Date: * Thu, Jun 20, 2019 02:54 PM
>>> *To: * "nijiaben"<nijiaben at perfma.com>; "David 
>>> Holmes"<david.holmes at oracle.com>; 
>>> "serviceability-dev"<serviceability-dev at openjdk.java.net>; 
>>> "jdk8u-dev"<jdk8u-dev at openjdk.java.net>; 
>>> "hotspot-runtime-dev"<hotspot-runtime-dev at openjdk.java.net>;
>>> *Subject: * Re: A Bug about the JVM Attach mechanism
>>> On 20/06/2019 05:10, nijiaben wrote:
>>>  > :
>>>  > I know this mechanism, can we provide means of recovery to avoid 
>>> unavailability caused by accidental deletion?
>>>  >
>>> Are you concerned about tmpreaper or cron jobs that periodically cleanup
>>> /tmp? There may indeed be an issue for applications that run for weeks
>>> or months. If someone is using jmap, jcmd or other tools using the
>>> attach API then it will trigger the attach listener to start. When they
>>> come back in a few weeks then the .java_pid<pid> file may have been
>>> removed so they cannot attach. Is this what what you are pointing out?
>>>
>>> -Alan
>