Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-8244847

Linux/PPC: runtime/CompressedOops/CompressedClassPointers: smallHeapTest fails

    XMLWordPrintable

    Details

    • Subcomponent:
    • Resolved In Build:
      b28
    • CPU:
      ppc
    • OS:
      linux

      Backports

        Description

        smallHeapTest fails on Linux/PPC, because the narrow klass base is not 0x0.

        ### Reproduce

        java -Xmx1g -Xshare:off -Xlog:metaspace*=info -version

        => Narrow klass base: 0x00007fff28000000 // Should be 0x0

        ### Preconditions

        CDS needs to be disabled.

        At most 2g maximum heap size.

        Linux kernel 4.12 or higher [6][7]
        Downported to 4.1.42 [8]

        ### Analysis (addresses modulo ASLR in the lower bits)

        JDK-8233410 made the java launcher a position independent executable (PIE). As a side effect the
        launcher gets loaded to 0x0000000100000000 (4g). Before it was loaded to 0x0000000010000000 (256m)
        [1]. With the precoditions above 4g is precisely the address where current heuristics do a one shot
        attempt to get a zero based compressed class space (CCS) which now always fails.

        On x86_64 the issue does not exist, because ELF_ET_DYN_BASE is well above 32g.

        More Details:

        The small java heap (-Xmx128m) is mapped just below 4g in
        ReservedHeapSpace::initialize_compressed_heap() which calls try_reserve_range() with highest_start
        == UnscaledOopHeapMax == 4g and lowest_start == HeapBaseMinAddress == 2g [2]. try_reserve_range()
        searches from top to bottom [3], and is successfull at first try. So the heap is mapped just below
        4g.

        Metaspace::global_initialize() requests to map the CCS right after the java heap [4]. This fails,
        because the launcher is already mapped to 4g (see above).
        Metaspace::reserve_address_space_for_compressed_classes() is called to handle the failure. On PPC this method does not try to get a zerobased CCS [5].

        [1] ELF_ET_DYN_BASE for PPC
            https://github.com/torvalds/linux/blob/79dede78c0573618e3137d3d8cbf78c84e25fabd/arch/powerpc/include/asm/elf.h#L27

        [2] ReservedHeapSpace::initialize_compressed_heap(): call to try_reserve_range(highest=4g, lowest=2g)
            http://hg.openjdk.java.net/jdk/jdk/file/dd1034e6350f/src/hotspot/share/memory/virtualspace.cpp#l563

        [3] ReservedHeapSpace::try_reserve_range(): search from top to bottom
            http://hg.openjdk.java.net/jdk/jdk/file/dd1034e6350f/src/hotspot/share/memory/virtualspace.cpp#l425

        [4] Metaspace::global_initialize(): try to map CCS just after java heap
            https://github.com/openjdk/jdk/blob/f2f3ba92d78da6af3bf7e379cad436bdfa3d29bc/src/hotspot/share/memory/metaspace.cpp#L703

        [5] Metaspace::reserve_address_space_for_compressed_classes() does not try to get a good address for CCS on PPC
            https://github.com/openjdk/jdk/blob/2f06893a29fba3b40cc1cf03095b75b217d1bb93/src/hotspot/share/memory/metaspace.cpp#L533

        [6] Linux kernel commit that changed ELF_ET_DYN_BASE from 0x20000000 to 0x100000000UL on PPC
            https://github.com/torvalds/linux/commit/47ebb09d54856500c5a5e14824781902b3bb738e

        [7] ELF_ET_DYN_BASE was changed [6] with Linux version 4.12
            https://github.com/torvalds/linux/blob/02445990a96e60a67526510d8b00f7e3d14101c3/Makefile

        [8] Downport of ELF_ET_DYN_BASE change to 4.1.42
            https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-4.1.y&id=cd5425dae2d687f50b7cac21f7b1eac431af8381

          Attachments

            Issue Links

              Activity

                People

                Assignee:
                rrich Richard Reingruber
                Reporter:
                rrich Richard Reingruber
                Votes:
                0 Vote for this issue
                Watchers:
                6 Start watching this issue

                  Dates

                  Created:
                  Updated:
                  Resolved: