RFR: Verifier should dump raw memory around the problematic oops

[Aleksey Shipilev]

When Verifier fails with broken oop, it is useful to see where the memory actually points. This
helps to catch off-by-one errors, see for example off-by-one error that points to fwdptr slot, not
to object header:

---------- 8< -----------------------------------------------------------------------------

# A fatal error has been detected by the Java Runtime Environment:
#
#  Internal Error
(/home/shade/trunks/shenandoah-jdk/src/hotspot/share/gc/shenandoah/shenandoahVerifier.cpp:62),
pid=41615, tid=41622
#  Error: Before Updating References, Marked; Object klass pointer must go to metaspace

Referenced from:
  no interior location recorded (probably a plain heap scan, or detached oop)

Object:
  0x00000000fb8813c0 - safe print, no details
  region: | 1905|R  |BTE     fb880000,     fb8ffea0,     fb900000|TAMS     fb880000,     fb880000|U
99%|T   0%|G  89%|S  10%|L  99%| |CP   0|SN            0,        0,     9634,     96f6


Raw heap memory:
0x00000000fb881390:   00000000 00000000 00000000 00000000
0x00000000fb8813a0:   00000000 00000000 00000000 00000000
0x00000000fb8813c0:   00000000 00000000 fb8813d0 00000000
0x00000000fb8813d0:   00000005 00000000 20002645 00000c44
0x00000000fb8813e0:   baadbabe baadbabe fb8813f0 00000000
0x00000000fb8813f0:   00000005 00000000 20002645 00000c45
0x00000000fb881400:   baadbabe baadbabe fb881410 00000000
0x00000000fb881410:   00000005 00000000 20002645 00000c46
0x00000000fb881420:   baadbabe baadbabe fb881430 00000000
0x00000000fb881430:   00000005 00000000 20002645 00000c47

---------- 8< -----------------------------------------------------------------------------

Thanks,
-Aleksey

[0]

diff -r d136c3d3b5c7 src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp	Wed May 16 08:44:23 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp	Wed May 16 18:47:58 2018 +0200
@@ -32,6 +32,27 @@
 #include "gc/shenandoah/shenandoahTraversalGC.hpp"
 #include "memory/resourceArea.hpp"

+void print_raw_memory(ShenandoahMessageBuffer &msg, void* loc) {
+  // Be extra safe. Only access data that is guaranteed to be safe:
+  // should be in heap, in known committed region, within that region.
+
+  ShenandoahHeap* heap = ShenandoahHeap::heap();
+  if (!heap->is_in(loc)) return;
+
+  ShenandoahHeapRegion* r = heap->heap_region_containing(loc);
+  if (r != NULL && r->is_committed()) {
+
+    address start = MAX2((address) r->bottom(), (address) loc - 32);
+    address end   = MIN2((address) r->end(),    (address) loc + 128);
+    if (start >= end) return;
+
+    stringStream ss;
+    os::print_hex_dump(&ss, start, end, 4);
+    msg.append("\n");
+    msg.append("Raw heap memory:\n%s", ss.as_string());
+  }
+}
+
 void ShenandoahAsserts::print_obj(ShenandoahMessageBuffer& msg, oop obj) {
   ShenandoahHeap* heap = ShenandoahHeap::heap();
   ShenandoahHeapRegion *r = heap->heap_region_containing(obj);
@@ -80,6 +101,7 @@
       stringStream ss;
       r->print_on(&ss);
       msg.append("  region: %s", ss.as_string());
+      print_raw_memory(msg, loc);
     }
   }
 }