diff --git a/contrib/jemalloc/include/jemalloc/jemalloc_FreeBSD.h b/contrib/jemalloc/include/jemalloc/jemalloc_FreeBSD.h
--- a/contrib/jemalloc/include/jemalloc/jemalloc_FreeBSD.h
+++ b/contrib/jemalloc/include/jemalloc/jemalloc_FreeBSD.h
@@ -39,7 +39,11 @@
 #  define JEMALLOC_TLS_MODEL	__attribute__((tls_model("initial-exec")))
 #endif
 #ifdef __amd64__
+#ifdef _USE_LG_VADDR_WIDE
+#  define LG_VADDR		64
+#else
 #  define LG_VADDR		48
+#endif
 #  define LG_SIZEOF_PTR		3
 #  define JEMALLOC_TLS_MODEL	__attribute__((tls_model("initial-exec")))
 #endif
diff --git a/lib/libc/stdlib/malloc/jemalloc/Makefile.inc b/lib/libc/stdlib/malloc/jemalloc/Makefile.inc
--- a/lib/libc/stdlib/malloc/jemalloc/Makefile.inc
+++ b/lib/libc/stdlib/malloc/jemalloc/Makefile.inc
@@ -5,6 +5,9 @@
 	test_hooks.c ticker.c tsd.c witness.c
 
 CFLAGS+=-I${SRCTOP}/contrib/jemalloc/include
+.if ${MK_JEMALLOC_LG_VADDR_WIDE} != no
+CFLAGS+=-D_USE_LG_VADDR_WIDE
+.endif
 
 .for src in ${JEMALLOCSRCS}
 MISRCS+=jemalloc_${src}
diff --git a/share/mk/src.opts.mk b/share/mk/src.opts.mk
--- a/share/mk/src.opts.mk
+++ b/share/mk/src.opts.mk
@@ -116,6 +116,7 @@
     IPFW \
     ISCSI \
     JAIL \
+    JEMALLOC_LG_VADDR_WIDE \
     KDUMP \
     KVM \
     LDNS \
diff --git a/tools/build/options/WITHOUT_JEMALLOC_LG_VADDR_WIDE b/tools/build/options/WITHOUT_JEMALLOC_LG_VADDR_WIDE
new file mode 100644
--- /dev/null
+++ b/tools/build/options/WITHOUT_JEMALLOC_LG_VADDR_WIDE
@@ -0,0 +1,5 @@
+Disallow programs to use more than 48 address bits on amd64.
+Incompatible with LA57 mode.
+Enabling this option might result in a slight reduction in memory
+consumption for jemalloc metadata, but also requires disabling LA57
+(if hardware supports it).