RFR: 8252500: ZGC on aarch64: Unable to allocate heap for certain Linux kernel configurations

[Christoph Göttschkes]

On Mon, 7 Sep 2020 09:29:47 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> The patch introduces a new function to probe for the highest valid bit in the virtual address space for userspace
>> programs on Linux.
>> I guarded the whole implementation to only probe on Linux, other platforms will remain unaffected. Possibly, it would
>> be nicer to move the probing code into an OS+ARCH specific source file. But since this is only a single function, I
>> thought it would be better to put it right next to the caller and guard it with an #ifdef LINUX.  The probing mechanism
>> uses a combination of msync + mmap, to first check if the address is valid using msync (if msync succeeds, the address
>> was valid). If msync fails, mmap is used to check if msync failed because the memory wasn't mapped, or if it failed
>> because the address is invalid. Due to some undefined behavior (documented in the msync man page), I also use a single
>> mmap at the end, if the msync approach failed before.  I tested msync with different combinations of mappings, and also
>> with sbrk, and it always succeeded, or failed with ENOMEM. I never got back any other error code.  The specified
>> minimum value has been chosen "randomly". The JVM terminates (unable to allocate heap), if this minimum value is
>> smaller than the requested Java Heap size, so it might be better to make the minimum dependent on the MaxHeapSize and
>> not a compile time constant? I didn't want to make the minimum too big, since for aarch64 on Linux, the documented
>> minimum would be 38 (see [1]).   I avoided MAP_FIXED_NOREPLACE, because according to the man page, it has been added in
>> Linux 4.17. There are still plenty of stable kernel versions around which will not have that feature, which means we
>> need to implement a workaround for it. Some of my test devices also have a kernel version lower than that.  I executed
>> the HotSpot tier1 JTreg tests on two different aarch64 devices. One with 4KB pages and 3 page levels and the other with
>> 4KB pages and 4 page levels. Tests passed on both devices.  [1]
>> https://www.kernel.org/doc/Documentation/arm64/memory.txt
>
> src/hotspot/cpu/aarch64/gc/z/zGlobals_aarch64.cpp line 183:
> 
>> 181:     void* result_addr = mmap((void*) high_addr, page_size, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1,
>> 0); 182:     if (result_addr != MAP_FAILED) {
>> 183:       max_address_bit = BitsPerSize_t - count_leading_zeros((size_t) result_addr) - 1;
> 
> It's not obvious to me that the '- 1' is correct here.

Let's assume the probing failed (for whatever reason) and we are actually able to allocate DEFAULT_MAX_ADDRESS_BIT:
00000000_00000000_10000000_00000000_00000000_00000000_00000000_00000000 = 1U << DEFAULT_MAX_ADDRESS_BIT
Mmap succeeds and returns exactly that address. Now the line would evaluate to the following:
64 - 16 - 1 = 47 <=> DEFAULT_MAX_ADDRESS_BIT

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PR: https://git.openjdk.java.net/jdk/pull/40