P4
In Java_java_nio_MappedByteBuffer_isLoaded0, we call mincore(2) to retrieve the paging status of an address range.
The size of the output buffer for mincore(2) depends on the number of pages in *system page size* in the given memory range (this is spelled out more or less explicitly on AIX and Linux, nothing is said on BSD/OSX, but I assume the same). The number of pages in the memory range is calculated by MappedByteBuffer.isLoaded() and handed down to Java_java_nio_MappedByteBuffer_isLoaded0() together with the memory range to test.
MappedByteBuffer.isLoaded() calculates this number of pages based on jjdk.internal.misc.Unsafe.pagesize(), which ultimately comes down to os::vm_page_size().
For AIX, os::vm_page_size() may return a page size larger than the system page size of 4K. The reason for this is that on AIX, memory can be backed by different page sizes, usually either 4K or 64K - e.g. posix thread stacks may have 4K pages, java heap (system V shared memory) with 64K pages, but mmap memory is always 4K page backed...
But as the OpenJDK code base generally assumes one homogeneous page size for everything - which is usually synonymous with os::vm_page_size() - a decision had to be made which page size to assume as a global system page size, and this may be a larger page size than the 4K system page size mincore(2) assumes.
This usually is no problem, but with mincore(2) it is: as the size of the output buffer depends on the number of pages, calculating with a too-large page size causes the output buffer to be too small and hence the buffer overflows. The solution must be to base the size of the mincore output buffer on the system page size.
The size of the output buffer for mincore(2) depends on the number of pages in *system page size* in the given memory range (this is spelled out more or less explicitly on AIX and Linux, nothing is said on BSD/OSX, but I assume the same). The number of pages in the memory range is calculated by MappedByteBuffer.isLoaded() and handed down to Java_java_nio_MappedByteBuffer_isLoaded0() together with the memory range to test.
MappedByteBuffer.isLoaded() calculates this number of pages based on jjdk.internal.misc.Unsafe.pagesize(), which ultimately comes down to os::vm_page_size().
For AIX, os::vm_page_size() may return a page size larger than the system page size of 4K. The reason for this is that on AIX, memory can be backed by different page sizes, usually either 4K or 64K - e.g. posix thread stacks may have 4K pages, java heap (system V shared memory) with 64K pages, but mmap memory is always 4K page backed...
But as the OpenJDK code base generally assumes one homogeneous page size for everything - which is usually synonymous with os::vm_page_size() - a decision had to be made which page size to assume as a global system page size, and this may be a larger page size than the 4K system page size mincore(2) assumes.
This usually is no problem, but with mincore(2) it is: as the size of the output buffer depends on the number of pages, calculating with a too-large page size causes the output buffer to be too small and hence the buffer overflows. The solution must be to base the size of the mincore output buffer on the system page size.