RFR (M) 8164921: Memory leaked when instrumentation.retransformClasses() is called repeatedly

[serguei.spitsyn at oracle.com]

Hi Coleen,

It looks good to me.
Some minor comments.

http://cr.openjdk.java.net/~coleenp/8164921.02/webrev/src/share/vm/memory/metaspace.cpp.frames.html

253 const static uint _small_block_max_size = 
sizeof(TreeChunk<Metablock, FreeList<Metablock> >)/HeapWordSize; 260 
assert(word_size >= _small_block_min_size, "There are no metaspace 
objects less than %u words", _small_block_min_size);

    Extra space before FreeList and after %u.

903 // Try small_blocks first
904 if (word_size < SmallBlocks::small_block_max_size()) {
905 // Don't create small_blocks() until needed.

    It makes sense to combine both comments. 
http://cr.openjdk.java.net/~coleenp/8164921.02/webrev/test/runtime/RedefineTests/RedefineLeak.java.html 


   38 import java.lang.NoSuchFieldException;
   39 import java.lang.NoSuchMethodException;

    These imports can be removed. Thanks, Serguei On 10/6/16 10:54, 
Coleen Phillimore wrote:
> Here is an update to the Metadata leak change.  There was a bug 
> introduced when cleaning this up, which Mikael also found. open webrev 
> at http://cr.openjdk.java.net/~coleenp/8164921.02/webrev Changes 
> include adding logging for report_metadata_oome, which necessitated 
> removing ResourceMarks in ClassLoaderData::dump because the stream 
> passed in already required a ResourceMark, so it got a nested 
> ResourceMark message for the stream. I changed logging for tracing 
> block allocations to log_trace(gc, metaspace, freelist, blocks). In 
> BlockFreelist::get_block and BlockFreelist::return_block() assert that 
> it's not called for a size smaller than the minimum allocation (which 
> was the bug).  Renamed get_raw_word_size() to 
> get_allocation_word_size().  This rounds up to the minimum allocation 
> size which is the same as small_block_min_size. Also, I added a test 
> that takes a long time to execute to verify this, and excluded it from 
> JPRT.  I could skip adding this test if people don't want it.  Also, 
> the test verifies that continuously redefining the same class gets 
> memory for the new class that was released because the block sizes are 
> the same.   When the test exits, it gets a metaspace OOM because 
> loading new classes and allocating metadata can't use the blocks 
> returned (wrong size). There is still fragmentation in this 
> implementation, but it's better that things deallocated < 12 words are 
> actually freed.  I'll file an RFE to work on a perfect algorithm, or 
> to investigate finding a better one, although I consider this a stress 
> test that uses all of metaspace to MaxMetaspaceSize, leaving 
> allocation only to the block fragments left.  This isn't a typical use 
> case. Some comments and corrections to my responses to Mikael below: 
> On 10/4/16 12:15 PM, Coleen Phillimore wrote:
>> Hi Mikael, Thanks for looking at this change. On 10/4/16 8:32 AM, 
>> Mikael Gerdin wrote:
>>> Hi Coleen, On 2016-09-30 21:02, Coleen Phillimore wrote:
>>>> Summary: Return Metablocks smaller than dictionary's dark matter. 
>>>> This change contributed by Jon Masamitsu and myself.  To reclaim 
>>>> "dark matter" this change adds an array of small blocks by size, 
>>>> created lazily, to return Metablocks smaller than the 
>>>> BinaryTreeDictionary entry's minimum size.   This change also fixed 
>>>> a bug in small object double free and adds debugging code to check 
>>>> for this case. With this change, the submitted test case runs 
>>>> indefinitely. Also passed rbt tier 1-5 testing. open webrev at 
>>>> http://cr.openjdk.java.net/~coleenp/8164921.01/webrev bug link 
>>>> https://bugs.openjdk.java.net/browse/JDK-8164921 
>>> I'd prefer it if SmallBlocks didn't expose its implementation by 
>>> returning its FreeLists by reference, could you change it to have * 
>>> return_chunk() * get_chunk() * num_chunks(word_size) and get rid of 
>>> list_at? 
>> Okay, I refactored this so small_chunks()->get_block() and 
>> return_block() are used rather than getting list_at.  I didn't see 
>> where you got num_chunks, but list_at is hidden.
>>> -- For how long do you plan to keep BlockFreelist::_all_blocks? I 
>>> see that it's debug only but I fear that it could case problems with 
>>> running out of native memory in our internal testing of debug builds. 
>> I thought about taking it out, but it helped me find the double free 
>> bug.   I think if we add new code to metadata and have to call 
>> deallocate_contents on it, we risk re-introducting these double free 
>> bugs.   I could take it out.  I don't think this gets that big but 
>> I'd hate to introduce some sort of OOM bug in our testing.
>>> BlockFreelist::min_size() is a bit of a misnomer since it returns 
>>> the minimum size of blocks to be put on the BlockTreeDictionary, not 
>>> the minimum size of blocks which are reusable. 
>> How about min_dictionary_size() ?
>>> Is there any particular reason behind _small_blocks being lazy 
>>> allocated and _dictionary not? 
>> We lazily create the BlockFreelists with this change.   // Lazily 
>> create a block_freelist   if (block_freelists() == NULL) {     
>> _block_freelists = new BlockFreelist();   } So the small_blocks are 
>> lazily created in the BlockFreelists but the dictionary is not.  I 
>> guess if we're going to create the BlockFreelists here, we'll most 
>> likely need both and maybe small_blocks need not be lazily created.  
>> Was that your suggestion? My concern with this change was all the 
>> space used by the small_blocks() but if we're doing any deallocation 
>> within the metaspace, at least one of the things will be <12 words. 
>> I'll make small_blocks() not be lazily allocated since BlockFreelist 
>> are.  These are pretty expensive, but should be limited to a few 
>> metaspaces. 
> Class SpaceManager doesn't need small_blocks() so I left small_blocks 
> as lazily allocated.
>>> I would prefer if BlockFreelist::return_block would perform the 
>>> checks in reverse order instead of having a return inside the first 
>>> if block, something like if (word_size > small_block_max_size) { 
>>> dict()->return_chunk(c) } else if (word_size > small_block_min_size) 
>>> { small_blocks()->return_chunk(c) } else {   // dark matter } 
>> Why?  We don't want to cast Metablock into dark matter so check if 
>> word_size < small_block_min_size first.   Metablock* free_chunk = 
>> ::new (p) Metablock(word_size);   if (word_size < 
>> SmallBlocks::small_block_max_size()) {     
>> small_blocks()->return_chunk(word_size);   } else {     
>> dictionary()->return_chunk(free_chunk);   } 
> There is no dark matter in these functions anymore.
>>> For BlockFreelist::get_block I realize that the code is a bit more 
>>> complex but this also raises a few questions. * When we allocate 
>>> from the dictionary we search for a block which is at least as large 
>>> as we ask for and split it, returning whatever was left back to the 
>>> free list. We don't search for "large enough" blocks from the small 
>>> blocks manager, is that intentional or just to keep the code simple 
>>> (I like simple)? 
>> I'm glad you asked about this so I could give background.   It turns 
>> out that we deallocate metaspace items better this way.  I had a 
>> version that did exactly what you said.  It was a simple sorted 
>> linked list of returned blocks < min_dictionary_size (12) where 
>> get_block returned the first block where the item would fit.  It had 
>> some best fit algorithm so if the block returned was a lot bigger, it 
>> wouldn't pick it. My implementation could get through 69300 
>> retransformations before the list didn't work anymore (too many small 
>> block fragments of the wrong size) and metaspace was exhausted 
>> (metaspace was limited to 12M in this test).  Jon's implementation 
>> ran this test indefinitely.  So it's somewhat simple but it worked 
>> really well.
>>> In the last part of get_block where we return the unused part of a 
>>> block retrieved from the dictionary uses compares with 
>>> BlockFreelist::min_size() which, as I mentioned above, is the min 
>>> size for blocks in the dictionary, not the min size for blocks to be 
>>> reusable. I think this code can just call return_block 
>>> unconditionally for any nonzero value of "unused" and let 
>>> return_block deal with dark matter. 
>> yes, I'll make that change. 
> This change has to be conditional because I assert that 
> BlockFreelist::return_block() is never called for < 
> small_block_min_size. Thanks, Coleen
>>> Are the changes to Method and ConstMethod the "bug fix to small 
>>> object double free"? 
>> Yes.
>>> Is the problem that when a method was freed its annotations were 
>>> deallocated as well? Could the annotations fields in the "old" 
>>> ConstMethod be cleared instead so that the old annotation arrays 
>>> could be kept or is that just needless complexity? 
>> I'd rather copy the annotations, because I don't know how the old 
>> method, which could still be running, might use the annotations. I 
>> don't want to mess with that but I see your point. Coleen
>>> Thanks /Mikael
>>>> Thanks, Coleen and Jon