RFR: (S): JDK-8200613: SA: jstack throws UnmappedAddressException with a CDS core file

Fri Dec 7 20:03:16 UTC 2018

Hi Jini,  We were just talking about this new option.  If someone gets a 
crash, I don't think they're going to run their application again with 
-XX:-DumpPrivateMappingsInCore in the case of the core file being too 
large.  So I don't know how generally useful this option is. I think it 
would be better to not add it and set the bit to include the mappings 
unconditionally.

How much larger is this core file (we had trouble understanding from the 
bug)?  If you need the mappings to understand and use the SA tools on 
it, we want to have them.

Thanks,
Coleen

On 12/7/18 2:22 PM, Jini George wrote:
> I have the revised webrev here:
>
> http://cr.openjdk.java.net/~jgeorge/8200613/webrev.02/index.html
>
> The extra changes here are to:
>
> * Introduce a new option DumpPrivateMappingsInCore to control the 
> dumping of the file backed private regions into the corefile.
>
> * Close the modules file before dumping core in os::abort(). 
> Currently, there is a small bug 
> (https://bugs.openjdk.java.net/browse/JDK-8215026) which prevents the 
> closure of the image file in unmapping the entire file.
>
> I plan to take up the unmapping of NIO MapMode.PRIVATE files as a 
> separate task (https://bugs.openjdk.java.net/browse/JDK-8215027) since 
> this seems a bit involved.
>
> Thanks a bunch,
> Jini.
>
> On 11/12/2018 10:26 AM, Jini George wrote:
>> Thank you very much, Chris, Kevin and Ioi for your comments!
>>
>> I will send another webrev with this change enabled under an opt-out 
>> flag, as you suggest, and would look at unmapping the JDK modules 
>> file and if possible, the NIO mapped files too in the signal handler.
>>
>> Thanks a bunch,
>> Jini.
>>
>> On 11/9/2018 11:53 PM, Ioi Lam wrote:
>>> Hi Jini,
>>>
>>> Thanks for investigating the size expansion issue.
>>>
>>> I agree that the size increase is worth it. Even when not using SA, 
>>> if we open the core file inside GDB, we cannot read certain sections 
>>> in the CDS archive (such as the RO section and strings sections). 
>>> That would make debugging difficult. So I am in favor of this change.
>>>
>>> For the JDK modules file, maybe we can unmap it in the signal 
>>> handler, before going ahead with the core dump? I think it's hardly 
>>> needed for debugging purposes. (Perhaps we can also do the same for 
>>> the NIO mapped files?)
>>>
>>> A opt-flag as suggested by Kevin is a good idea.
>>>
>>> Thanks
>>>
>>> - Ioi
>>>
>>> On 11/9/18 3:29 AM, Kevin Walls wrote:
>>>> Hi Jini,
>>>>
>>>> Looks good to me.  It might be a significant increase in size of 
>>>> _some_ core files, but so many core files we see are much larger, 
>>>> in gigabytes++ of course, so the CDS data size should not be such a 
>>>> significant increase on (I think) most files.
>>>>
>>>> The flexibiity of always having the CDS data there is very 
>>>> significant.  A core file should ideally be usable, without 
>>>> additionally requiring the CDS archive from the machine. That 
>>>> additional human round-trip upload request on every transmitted 
>>>> core that needs investigating, seems like a less efficient route...).
>>>>
>>>> Is there an opt-out?  It's conditional on UseSharedSpaces but could 
>>>> there be a flag to disable, in case we see crashes with gigabytes 
>>>> of private mappings that we really don't want to retain (the user 
>>>> would have to know to set a flag, to disable the new coredump 
>>>> filter ahead of time).
>>>>
>>>> Thanks!
>>>> Kevin
>>>>
>>>>
>>>> On 29/10/2018 06:02, Jini George wrote:
>>>>> Thank you very much, Ioi, for looking into this, and the 
>>>>> clarification offline. My bad, I had missed the earlier mail from 
>>>>> you. :-( My responses below.
>>>>>
>>>>> Yes, I had tested this on MacOS. The issue does not exist on MacOS 
>>>>> since the file backed private mmap()-ed regions get dumped into 
>>>>> the MacOS corefiles by default.
>>>>>
>>>>> The corefile sizes on Linux do increase due to this change. And 
>>>>> the increase would also include any file mapped using NIO with 
>>>>> MapMode.PRIVATE. The typical corefile size increase with this 
>>>>> change would include the following components at a high level:
>>>>>
>>>>> * Any NIO file mapping with MapMode.PRIVATE.
>>>>> * Any file mmap()-ed by any native library with MAP_PRIVATE.
>>>>> * The read only CDS regions (ro and od): Of the order of a few MB.
>>>>> * The shared strings CDS region. (typically less than 1 MB).
>>>>> * 2 MB per native shared library (regions with ---p permissions 
>>>>> mapped by the dynamic linker for better alignment and for keeping 
>>>>> libraries efficiently shareable).
>>>>> * The JDK 'modules' file. (About 140 MB).
>>>>>
>>>>> So, without including any NIO mapping, I typically see around 
>>>>> 250-300 MB increase in the corefile sizes. I agree that the size 
>>>>> increase could be a cause for concern, but for FWIW, these 
>>>>> privately mapped files get dumped into the corefile for MacOS too. 
>>>>> And the corefile sizes for the same program on MacOS are way 
>>>>> larger (of the order of a few GB as against about 300 MB on Linux 
>>>>> (without the change)).
>>>>>
>>>>> The advantage of fixing this by modifying the coredump_filter v/s 
>>>>> doing it in SA (by reading in more sections of the shared archive 
>>>>> file) is that this would benefit other debuggers like gdb also. 
>>>>> (And reduces the dependence on having the shared archive file 
>>>>> being available at the time of debugging). If folks still think 
>>>>> this is a cause for concern, I could make modifications to fix 
>>>>> this by reading in the regions from the shared archive file in the 
>>>>> SA code. I also wonder if it is worth taking a relook at the 
>>>>> mapping types of the various CDS regions also.
>>>>>
>>>>> Thank you,
>>>>> Jini.
>>>>>
>>>>> On 10/22/2018 10:27 AM, Ioi Lam wrote:
>>>>>> Hi Jini,
>>>>>>
>>>>>> Did you see my earlier reply? I might have sent it out during the 
>>>>>> mail server outage days :-(
>>>>>>
>>>>>> http://openjdk.5641.n7.nabble.com/RFR-S-JDK-8200613-SA-jstack-throws-UnmappedAddressException-with-a-CDS-core-file-td352681.html#a353026 
>>>>>>
>>>>>>
>>>>>> Here was my reply again:
>>>>>>
>>>>>>> Hi Jini,
>>>>>>>
>>>>>>> The changes looks good to me.
>>>>>>>
>>>>>>> Have you tested this on MacOS? CDS heap support is also enabled on
>>>>>>> MacOS. See macros.hpp:
>>>>>>>
>>>>>>>      #if INCLUDE_CDS && INCLUDE_G1GC && defined(_LP64) && 
>>>>>>> !defined(_WINDOWS)
>>>>>>>      #define INCLUDE_CDS_JAVA_HEAP 1
>>>>>>>
>>>>>>> Also, besides CDS, do we know how often other files will be 
>>>>>>> mmaped with
>>>>>>> MAP_PRIVATE? Will users get huge core files because CDS is 
>>>>>>> enabled? In
>>>>>>> JDK 12, CDS will be enabled by default (see JDK-8202951), so all 
>>>>>>> users
>>>>>>> will be affected by the following line:
>>>>>>>
>>>>>>>    if (UseSharedSpaces) {
>>>>>>>      set_coredump_filter(FILE_BACKED_PVT_BIT);
>>>>>>>    }
>>>>>>>
>>>>>>> Maybe you can run an big app such as Eclipse, trigger a core 
>>>>>>> dump, and
>>>>>>> compare the size of the core file before/after this change?
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>> - Ioi 
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> - Ioi
>>>>>>
>>>>>>
>>>>>> On 10/21/18 8:58 PM, Jini George wrote:
>>>>>>> Gentle reminder!
>>>>>>>
>>>>>>> Thanks,
>>>>>>> - Jini
>>>>>>>
>>>>>>> On 10/9/2018 11:31 AM, Jini George wrote:
>>>>>>>> Hello!
>>>>>>>>
>>>>>>>> [Including runtime-dev since the changes are in runtime code]
>>>>>>>>
>>>>>>>> Requesting reviews for:
>>>>>>>>
>>>>>>>> Webrev: http://cr.openjdk.java.net/~jgeorge/8200613/webrev.01/
>>>>>>>> BugID: https://bugs.openjdk.java.net/browse/JDK-8200613
>>>>>>>>
>>>>>>>> Issue: jhsdb jstack would throw an UnmappedAddressException 
>>>>>>>> with a core file generated from a CDS enabled java process. 
>>>>>>>> This is seen only with Linux and with G1GC, while trying to 
>>>>>>>> read in data from the shared strings region (the closed archive 
>>>>>>>> heap space). This region (which is a file backed private memory 
>>>>>>>> region) is not dumped into the corefile for Linux. This, being 
>>>>>>>> a heap region (and therefore being a read-write region) is also 
>>>>>>>> not read in from the classes.jsa file in SA since only the read 
>>>>>>>> only regions are read in while processing the core file. (The 
>>>>>>>> expectation being that the read write regions are in the core 
>>>>>>>> file).
>>>>>>>>
>>>>>>>> Proposed solution: The proposed solution is to have the 
>>>>>>>> coredump_filter value corresponding to the CDS process to 
>>>>>>>> include bit 2 (file-backed private memory), so that the 
>>>>>>>> file-backed private memory region also gets dumped into the 
>>>>>>>> corefile. The proposed fix is in 
>>>>>>>> src/hotspot/os/linux/os_linux.cpp.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Jini.
>>>>>>>>
>>>>>>
>>>>
>>>