Request for review (S): 7102044 G1: VM crashes with assert(old_end != new_end) failed: don't call this otherwise

[Vladimir Kozlov]

Bengt Rutisson wrote:
>> I am fine with SIZE_MAX if it passed JPRT (including embedded) as you 
>> said. If you can, please, verify if it exist in bsd and mac. May be 
>> Dan or Christian can help.
> 
> I'll check with them. You mean Dan Daugherty and Christian Thalenger, 
> right?

Yes, them.

>> Did you measure the additional total time they added to JPRT job? If 
>> it is around 5 mins I am fine with it.
> 
> Here is a run I did where I only run the internalvmtests target. Most of 
> them take 1-2 minutes. windows_x64 takes 3.5 minutes.
> http://prt-web.us.oracle.com//archive/2011/10/2011-10-27-124029.brutisso.hs-gc-max-array-length//JobStatus.txt 

Nice.

Thanks,
Vladimir

> 
> 
> Thanks again for the review!
> Bengt
> 
>>
>> Thanks,
>> Vladimir
>>
>> On 10/27/11 5:37 AM, Bengt Rutisson wrote:
>>>
>>> Hi again,
>>>
>>> An updated webrev based on comments from Stefan and Vladimir
>>> http://cr.openjdk.java.net/~brutisso/7102044/webrev.02/
>>>
>>> Bengt
>>>
>>> On 2011-10-27 13:44, Bengt Rutisson wrote:
>>>>
>>>> Hi Vladimir,
>>>>
>>>> Thanks for looking at this change!
>>>>
>>>> On 2011-10-26 17:44, Vladimir Kozlov wrote:
>>>>> Hi Bengt,
>>>>>
>>>>> I thought VM should throw OOM exception in such cases (as your 
>>>>> example) since you can't allocate such arrays in
>>>>> 32-bit VM. Does the size check for OOM happened after heap resize 
>>>>> calculation?
>>>>
>>>> You are right. The VM will throw an OOME both before and after my 
>>>> change. But for different reasons.
>>>>
>>>> With my change we will detect already in 
>>>> typeArrayKlass::allocate_common() that length <= max_length() and 
>>>> throw an
>>>> OOME. Before this change we could pass that test and go on in the 
>>>> allocation path. This would result in a size_t
>>>> overflow when we want to expand the heap. We would do a lot of extra 
>>>> work to expand to the wrong size. Then the
>>>> allocation would fail and we would throw OOME.
>>>>
>>>> I think it is better to detect this early and avoid the extra work. 
>>>> This way the assert that G1 does (which I think
>>>> makes sense) will hold. We already have the length check so I assume 
>>>> that this was the intention from the beginning.
>>>>
>>>>> Is SIZE_MAX defined on all platforms?
>>>>
>>>> I did try to look around the hotspot code for any uses of max size_t 
>>>> but I could not find it. So, I reverted to
>>>> looking at standard ways of doing it. SIZE_MAX seems to be defined 
>>>> on all our platforms, since my code passes JPRT.
>>>> And it seems to be part of the C99 standard:
>>>>
>>>> http://en.wikipedia.org/wiki/Size_t
>>>>
>>>> I thought it would be better to rely on this than for example 
>>>> "(size_t)-1". But I am happy to use any other define.
>>>>
>>>>> You still have problem with short[] and char[] since 
>>>>> 2*max_jint+header_size will overflow.
>>>>
>>>> Right. I did add some unit tests to catch this (but as you point out 
>>>> below those tests did not detect this overflow).
>>>> Thanks for catching it. I will fix it and send out an updated webrev.
>>>>
>>>>> Also max_array_length() puts unneeded limitation on double[] and 
>>>>> long[] max length in 64 but VM, it should be
>>>>> max_jint. I think we can do something like next:
>>>>>
>>>>> const size_t max_words_per_size_t = 
>>>>> align_size_down((SIZE_MAX/HeapWordSize - header_size(type)), 
>>>>> MinObjAlignment);
>>>>> const size_t max_elements_per_size_t = HeapWordSize * 
>>>>> max_words_per_size_t / type2aelembytes(type);
>>>>> if ((size_t)max_jint < max_elements_per_size_t);
>>>>> return max_jint ;
>>>>> return (int32_t)max_elements_per_size_t;
>>>>
>>>> I did not intend for my change to limit anything for 64 bits. That's 
>>>> why I added the MIN2((size_t)max_jint,
>>>> max_words_per_size_t) statement and included the old calculation in 
>>>> the unit tests to verify that nothing has changed
>>>> on 64 bits.
>>>>
>>>> I can change the code as you suggest, but I would like to understand 
>>>> what the problem is with my code first. I think
>>>> it handles 64 bits correctly.
>>>>
>>>>> check_overflow() should use julong type for expressions instead of 
>>>>> size_t to catch overflow:
>>>>>
>>>>> +bool arrayOopDesc::check_overflow(BasicType type) {
>>>>> + julong length = (julong)max_array_length(type);
>>>>> + julong bytes_per_element = (julong)type2aelembytes(type);
>>>>> + julong bytes = length * bytes_per_element + 
>>>>> (julong)header_size_in_bytes();
>>>>> + return (julong)(size_t)bytes == bytes;
>>>>> +}
>>>>
>>>> Right. This is why I didn't catch the problem above. Thanks. I'll fix.
>>>>
>>>>> Rename check_overflow() to check_max_length_overflow() or something.
>>>>
>>>> Done.
>>>>
>>>>> I don't think we need incorrect old_max_array_length() method and 
>>>>> corresponding checks for LP64.
>>>>
>>>> Ok. I'll keep it for some more testing to make sure that I don't 
>>>> unintentionally change anything for 64 bit. But I'll
>>>> remove it before I push.
>>>>
>>>>> Initialization and execution of additional JPRT tests will increase 
>>>>> jobs time. I think you could add InternalVMTests
>>>>> to existing client and server tests:
>>>>>
>>>>> < $(PRODUCT_HOME)/bin/java $(JAVA_OPTIONS) -version
>>>>> ---
>>>>> > $(PRODUCT_HOME)/bin/java $(JAVA_OPTIONS) 
>>>>> -XX:+IgnoreUnrecognizedVMOptions -XX:+ExecuteInternalVMTests -version
>>>>>
>>>>
>>>> I see your point regarding the increased job times. But I think it 
>>>> is much clearer to have the internal unit tests as
>>>> a separate target. That way they can be included and excluded with a 
>>>> JPRT command line.
>>>>
>>>> Also, I don't really like the IgnoreUnrecognizedVMOptions solution. 
>>>> I would like to be more explicit about that this
>>>> is only a (fast)debug build flag.
>>>>
>>>> Thanks,
>>>> Bengt
>>>>
>>>>> regards,
>>>>> Vladimir
>>>>>
>>>>> On 10/26/11 6:18 AM, Bengt Rutisson wrote:
>>>>>>
>>>>>> Hi all,
>>>>>>
>>>>>> Can I get a couple of reviews to this fairly small change?
>>>>>> http://cr.openjdk.java.net/~brutisso/7102044/webrev.01/
>>>>>>
>>>>>> Sorry for the wide distribution, but the bug is reported on GC, I 
>>>>>> think the actual change (in arrayOop.hpp) is runtime
>>>>>> responsibility and the method that is being changed is used on a 
>>>>>> few occasions in C2. So, I figured I'd like
>>>>>> everyone to
>>>>>> know that this is about to go in.
>>>>>>
>>>>>> CR:
>>>>>> 7102044 G1: VM crashes with assert(old_end != new_end) failed: 
>>>>>> don't call this otherwise
>>>>>> http://monaco.sfbay.sun.com/detail.jsf?cr=7102044
>>>>>>
>>>>>> Background
>>>>>> arrayOopDesc::max_array_length() returns the maximum length an 
>>>>>> array can have based on the type of the array elements.
>>>>>> This is passed on through the allcocation path as a word size. 
>>>>>> When we need to expand the heap we have to convert the
>>>>>> word size to a byte size. The problem is that on 32 bit platforms 
>>>>>> this conversion may overflow a size_t. Which will
>>>>>> result in that we call expand() with a much smaller size than 
>>>>>> required size or even with a size of 0.
>>>>>>
>>>>>> Here is a small reproducer that will trigger the assert mentioned 
>>>>>> in the CR if we run it with -XX:+UseG1GC:
>>>>>>
>>>>>> public class LargeObj {
>>>>>> public static void main(String[] args) {
>>>>>> int[] a = new int[2147483639];
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> It is not only G1 that has this problem. ParallelGC has the same 
>>>>>> issue, it just doesn't have an assert to detect it. It
>>>>>> can be argued that the GCs should check for overflows, but I think 
>>>>>> that since arrayOopDesc::max_array_length() will
>>>>>> catch the problem earlier and solve it for all GCs this is a 
>>>>>> better place to fix the problem.
>>>>>>
>>>>>> I added some unit tests to test the new implementation. To make 
>>>>>> sure that these tests are run I had to update some more
>>>>>> files. The actual change is in arrayOop.hpp. The rest of the files 
>>>>>> have just been changed to allow the unit tests to
>>>>>> run.
>>>>>>
>>>>>> A few questions:
>>>>>>
>>>>>> (1) In arrayOopDesc::max_array_length() I use MIN2 to make sure 
>>>>>> that we don't overflow a size_t. This is only needed on
>>>>>> 32 bit platforms. I prefer this general code compared to #ifndef 
>>>>>> _LP64. Any thoughts?
>>>>>>
>>>>>> (2) I made the unit tests run as JPRT make targets. I guess it can 
>>>>>> also be done as jtreg tests, but that way it would
>>>>>> only be executed for PIT etc. Any thoughts on this?
>>>>>>
>>>>>> Thanks,
>>>>>> Bengt
>>>>
>>>
>