RFR: Verifier should dump raw memory around the problematic oops

[Roman Kennke]

This looks very useful in some situations. Please push it!

Roman


> 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);
>      }
>    }
>  }
>