diff --git a/src/cmd/ksh93/tests/leaks.sh b/src/cmd/ksh93/tests/leaks.sh
index 5af1f5b3f..256cb4644 100755
--- a/src/cmd/ksh93/tests/leaks.sh
+++ b/src/cmd/ksh93/tests/leaks.sh
@@ -23,6 +23,7 @@
 
 # Determine method for running tests.
 # The 'vmstate' builtin can be used if ksh was compiled with vmalloc.
+# (Pass -D_AST_vmalloc in CCFLAGS; for testing only as it's deprecated)
 if	builtin vmstate 2>/dev/null &&
 	n=$(vmstate --format='%(busy_size)u') &&
 	let "($n) == ($n) && n > 0"	# non-zero number?
diff --git a/src/lib/libast/features/vmalloc b/src/lib/libast/features/vmalloc
index 177d3e6fb..ba68abd5a 100644
--- a/src/lib/libast/features/vmalloc
+++ b/src/lib/libast/features/vmalloc
@@ -57,9 +57,6 @@ tst	map_malloc note{ map malloc to _ast_malloc }end noexecute{
 	extern void* calloc _ARG_((unsigned int, unsigned int));
 	#define LOCAL()	calloc(1,1)
 	#endif
-	#if __CYGWIN__
-	#define extern __declspec(dllexport)
-	#endif
 	#define HT double
 	static HT heap[1024 * 4];
 	static HT* hp = &heap[1];
@@ -207,9 +204,13 @@ tst	malloc_hook note{ gnu malloc hooks work }end execute{
 
 cat{
 	#include "FEATURE/mmap"
-	#if _AST_ksh_release || _BLD_INSTRUMENT || cray || _UWIN && _BLD_ast
+	/* AST vmalloc is deprecated for being crashy and wasteful; ksh 93u+m uses the OS's malloc by default */
+	#undef _std_malloc
+	#undef _AST_std_malloc
+	#if !_AST_vmalloc
 	#undef	_map_malloc
 	#define _std_malloc	1	/* defer to standard malloc */
+	#define _AST_std_malloc	1
 	#endif
 	#if _mmap_anon
 	#define _mem_mmap_anon	1
diff --git a/src/lib/libast/man/vmalloc.3 b/src/lib/libast/man/vmalloc.3
index 75d4f585f..e9c60b99e 100644
--- a/src/lib/libast/man/vmalloc.3
+++ b/src/lib/libast/man/vmalloc.3
@@ -66,6 +66,13 @@ vmalloc \- virtual memory allocation
 .MW "Void_t* valloc(size_t size);"
 .MW "int setregmax(int regmax);"
 .fi
+.SH DEPRECATION NOTICE
+In the 2021 ksh 93u+m distribution, \fIvmalloc\fP(3) was deprecated for
+being unstable and wasteful in comparison to \fImalloc\fP(3) implementations
+that come with modern operating systems.
+By default, the interfaces described here map to the OS's standard
+\fImalloc\fP(3) implementation. \fIvmalloc\fP can be enabled at compile time
+by passing \fB-D_AST_vmalloc\fP in \fBCCFLAGS\fP.
 .SH DESCRIPTION
 These functions for dynamic storage allocation work in
 \fIregions\fP of memory.
diff --git a/src/lib/libast/vmalloc/malloc.c b/src/lib/libast/vmalloc/malloc.c
index 8abbc0c37..090d10b83 100644
--- a/src/lib/libast/vmalloc/malloc.c
+++ b/src/lib/libast/vmalloc/malloc.c
@@ -85,10 +85,6 @@ typedef struct ______mstats Mstats_t;
  * will simply call malloc etc.
  */
 
-#if !defined(_AST_std_malloc) && __CYGWIN__
-#define _AST_std_malloc	1
-#endif
-
 /*	malloc compatibility functions
 **
 **	These are aware of debugging/profiling and are driven by the
diff --git a/src/lib/libcmd/vmstate.c b/src/lib/libcmd/vmstate.c
index d0b16f281..92381c9b6 100644
--- a/src/lib/libcmd/vmstate.c
+++ b/src/lib/libcmd/vmstate.c
@@ -21,6 +21,12 @@
 ***********************************************************************/
 #pragma prototyped
 
+#include <cmd.h>
+#include <vmalloc.h>
+#include <sfdisc.h>
+
+#if !_std_malloc  /* do not pointlessly compile this if vmalloc is disabled */
+
 #define FORMAT		"region=%(region)p method=%(method)s flags=%(flags)s size=%(size)d segments=%(segments)d busy=(%(busy_size)d,%(busy_blocks)d,%(busy_max)d) free=(%(free_size)d,%(free_blocks)d,%(free_max)d)"
 
 static const char usage[] =
@@ -49,10 +55,6 @@ static const char usage[] =
 "[+SEE ALSO?\bvmalloc\b(3)]"
 ;
 
-#include <cmd.h>
-#include <vmalloc.h>
-#include <sfdisc.h>
-
 typedef struct State_s
 {
 	char*		format;
@@ -200,3 +202,5 @@ b_vmstate(int argc, char** argv, Shbltin_t* context)
 	}
 	return 0;
 }
+
+#endif /* !_std_malloc */