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

Jini George jini.george at oracle.com
Mon Oct 29 06:02:15 UTC 2018

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,

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.

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