9525 kmem_dump_size is a corrupting influence

@@ -19,11 +19,11 @@
  * CDDL HEADER END
  */
 
 /*
  * Copyright (c) 1998, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright 2016 Joyent, Inc.
+ * Copyright 2018 Joyent, Inc.
  */
 
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/systm.h>

@@ -72,10 +72,12 @@
 #include <sys/clock_impl.h>
 #include <sys/hold_page.h>
 
 #include <bzip2/bzlib.h>
 
+#define ONE_GIG (1024 * 1024 * 1024UL)
+
 /*
  * Crash dump time is dominated by disk write time.  To reduce this,
  * the stronger compression method bzip2 is applied to reduce the dump
  * size and hence reduce I/O time.  However, bzip2 is much more
  * computationally expensive than the existing lzjb algorithm, so to

@@ -136,11 +138,11 @@
 #define MINCPU_NOT_SET  ((uint_t)-1)
 uint_t dump_plat_mincpu = MINCPU_NOT_SET;
 
 /* tunables for pre-reserved heap */
 uint_t dump_kmem_permap = 1024;
-uint_t dump_kmem_pages = 8;
+uint_t dump_kmem_pages = 0;
 
 /* Define multiple buffers per helper to avoid stalling */
 #define NCBUF_PER_HELPER        2
 #define NCMAP_PER_HELPER        4
 

@@ -677,15 +679,27 @@
                 new->maxvm = vmem_xalloc(heap_arena, new->maxvmsize,
                     CBUF_MAPSIZE, 0, 0, NULL, NULL, VM_SLEEP);
         }
 
         /*
-         * Reserve memory for kmem allocation calls made during crash
-         * dump.  The hat layer allocates memory for each mapping
-         * created, and the I/O path allocates buffers and data structs.
-         * Add a few pages for safety.
+         * Reserve memory for kmem allocation calls made during crash dump.  The
+         * hat layer allocates memory for each mapping created, and the I/O path
+         * allocates buffers and data structs.
+         *
+         * On larger systems, we easily exceed the lower amount, so we need some
+         * more space; the cut-over point is relatively arbitrary.  If we run
+         * out, the only impact is that kmem state in the dump becomes
+         * inconsistent.
          */
+
+        if (dump_kmem_pages == 0) {
+                if (physmem > (16 * ONE_GIG) / PAGESIZE)
+                        dump_kmem_pages = 20;
+                else
+                        dump_kmem_pages = 8;
+        }
+
         kmem_dump_init((new->ncmap * dump_kmem_permap) +
             (dump_kmem_pages * PAGESIZE));
 
         /* set new config pointers */
         *old = *new;