RFR: JDK-8281023: NMT integration into pp debug command does not work

[Thomas Stuefe]

JDK-8280289 enhanced the debug pp() command to use NMT if enabled, and to print NMT related info. That is useful, but there are some issues.

On debug, it just asserts, since the empty reserved region we create to hold the output of the mmap-search is created with address=NULL:


(gdb) call pp(0x7ffff010b030)

"Executing pp"

Thread 2 "java" received signal SIGSEGV, Segmentation fault.
0x00007ffff6721a71 in VirtualMemoryRegion::VirtualMemoryRegion (this=this at entry=0x7ffff5bb2620, addr=addr at entry=0x0, size=size at entry=0) at /shared/projects/openjdk/jdk-jdk/source/src/hotspot/share/services/virtualMemoryTracker.hpp:180
180 assert(addr != NULL, "Invalid address");


On release we don't assert and get further, but the use of SafeFetch is slightly wrong. It will deny us any NMT data about p if *p==0:


if (CanUseSafeFetchN() && SafeFetchN((intptr_t*)p, 0) != 0) {


This patch:
- fixes uses of SafeFetch
- changes the mmap-region-search-code to not require an empty ReservedMemoryRegion in order to avoid triggering the assert in virtualMemoryTracker.hpp:180
- adds a comment about the safe use of pp() in gdb (one needs to switch off signal handling of SIGSEGV for this to work)

Tests:
- I tested manually that pp works with different levels of NMT (Linux x64)
- GHAs in process

-------------

Commit messages:
 - Fix NMT integration into pp() debug command

Changes: https://git.openjdk.java.net/jdk/pull/7297/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7297&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281023
  Stats: 28 lines in 3 files changed: 9 ins; 3 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7297.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7297/head:pull/7297

PR: https://git.openjdk.java.net/jdk/pull/7297