code review 69870055: runtime: fix runaway memory usage

src/pkg/runtime/mgc0.c

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 // Garbage collector (GC).
 //
 // GC is:
 // - mark&sweep
 // - mostly precise (with the exception of some C-allocated objects, assembly frames/arguments, etc)
 // - parallel (up to MaxGcproc threads)
@@ skipping 2328 matching lines @@
 
         t4 = runtime·nanotime();
         mstats.last_gc = t4;
         mstats.pause_ns[mstats.numgc%nelem(mstats.pause_ns)] = t4 - t0;
         mstats.pause_total_ns += t4 - t0;
         mstats.numgc++;
         if(mstats.debuggc)
                 runtime·printf("pause %D\n", t4-t0);
 
         if(runtime·debug.gctrace) {
+                heap1 = mstats.heap_alloc;
                 updatememstats(&stats);
-                heap1 = mstats.heap_alloc;
+                if(heap1 != mstats.heap_alloc) {
+                        runtime·printf("runtime: mstats skew: heap=%p/%p\n", heap1, mstats.heap_alloc);
+                        runtime·throw("mstats skew");
+                }
                 obj = mstats.nmalloc - mstats.nfree;
 
                 stats.nprocyield += work.markfor->nprocyield;
                 stats.nosyield += work.markfor->nosyield;
                 stats.nsleep += work.markfor->nsleep;
 
                 runtime·printf("gc%d(%d): %D+%D+%D ms, %D -> %D MB, %D (%D-%D) objects,"
                                 " %d/%d/%d sweeps,"
                                 " %D(%D) handoff, %D(%D) steal, %D/%D/%D yields\n",
                         mstats.numgc, work.nproc, (t3-t2)/1000000, (t2-t1)/1000000, (t1-t0+t4-t3)/1000000,
@@ skipping 372 matching lines @@
 
         n = (h->arena_used - h->arena_start) / wordsPerBitmapWord;
         n = ROUND(n, bitmapChunk);
         n = ROUND(n, PhysPageSize);
         if(h->bitmap_mapped >= n)
                 return;
 
         runtime·SysMap(h->arena_start - n, n - h->bitmap_mapped, &mstats.gc_sys);
         h->bitmap_mapped = n;
 }