* mvapi's deregister call calls free, and the fast bins in ptmalloc2 aren't

reentrant for free(), so we can't call free() from inside an sbrk() handler.
  The solution is never call sbrk() with a negative number.  The mmap() allocator
  used for large allocations does not have this problem and continues to give
  memory back to the OS as soon as possible.

This should go to both the v1.1 and v1.0 branches.

This commit was SVN r9943.

opal/mca/memory/ptmalloc2/malloc.c

@@ -62,6 +62,10 @@ extern void*  opal_mem_free_ptmalloc2_mmap(void *start, size_t length,
    are never reused, so this keeps the number of calls to munmap and
    sbrk down significantly */
 #define DEFAULT_MMAP_THRESHOLD (2*1024*1024)
+/* free() of small allocations is not reentrant and the mvapi libraries
+   call free() from deregister.  Big allocations (using mmap()) are fine
+   and don't need any special care. */
+#define MORECORE_CANNOT_TRIM 1
 
 /* make some non-GCC compilers happy */
 #ifndef __GNUC__
@@ -2385,12 +2389,16 @@ static void malloc_init_state(av) mstate av;
 
 #if __STD_C
 static Void_t*  sYSMALLOc(INTERNAL_SIZE_T, mstate);
+#ifndef MORECORE_CANNOT_TRIM
 static int      sYSTRIm(size_t, mstate);
+#endif
 static void     malloc_consolidate(mstate);
 static Void_t** iALLOc(mstate, size_t, size_t*, int, Void_t**);
 #else
 static Void_t*  sYSMALLOc();
+#ifndef MORECORE_CANNOT_TRIM
 static int      sYSTRIm();
+#endif
 static void     malloc_consolidate();
 static Void_t** iALLOc();
 #endif
@@ -3255,6 +3263,7 @@ static Void_t* sYSMALLOc(nb, av) INTERNAL_SIZE_T nb; mstate av;
 }
 
 
+#ifndef MORECORE_CANNOT_TRIM
 /*
   sYSTRIm is an inverse of sorts to sYSMALLOc.  It gives memory back
   to the system (via negative arguments to sbrk) if there is unused
@@ -3323,6 +3332,7 @@ static int sYSTRIm(pad, av) size_t pad; mstate av;
   }
   return 0;
 }
+#endif
 
 #ifdef HAVE_MMAP