diff --git a/src/include/stdlib.h b/src/include/stdlib.h
index b0f3dd14..7a26ed90 100644
--- a/src/include/stdlib.h
+++ b/src/include/stdlib.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: stdlib.h,v 1.74 2018/11/18 16:15:18 otto Exp $	*/
+/*	$OpenBSD: stdlib.h,v 1.75 2018/11/21 06:57:04 otto Exp $	*/
 /*	$NetBSD: stdlib.h,v 1.25 1995/12/27 21:19:08 jtc Exp $	*/
 
 /*-
@@ -112,7 +112,6 @@ long	 labs(long);
 ldiv_t	 ldiv(long, long);
 void	*malloc(size_t);
 #if __BSD_VISIBLE
-size_t	 malloc_usable_size(void *);
 void	freezero(void *, size_t)
 		 __attribute__ ((__bounded__(__buffer__,1,2)));
 void	*reallocarray(void *, size_t, size_t);
diff --git a/src/lib/libc/stdlib/malloc.3 b/src/lib/libc/stdlib/malloc.3
index c1776f61..c0472ee2 100644
--- a/src/lib/libc/stdlib/malloc.3
+++ b/src/lib/libc/stdlib/malloc.3
@@ -30,9 +30,9 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: malloc.3,v 1.119 2018/11/18 16:15:18 otto Exp $
+.\"	$OpenBSD: malloc.3,v 1.120 2018/11/21 06:57:04 otto Exp $
 .\"
-.Dd $Mdocdate: November 18 2018 $
+.Dd $Mdocdate: November 21 2018 $
 .Dt MALLOC 3
 .Os
 .Sh NAME
@@ -43,8 +43,7 @@
 .Nm reallocarray ,
 .Nm recallocarray ,
 .Nm freezero ,
-.Nm aligned_alloc ,
-.Nm malloc_usable_size
+.Nm aligned_alloc
 .Nd memory allocation and deallocation
 .Sh SYNOPSIS
 .In stdlib.h
@@ -65,7 +64,6 @@
 .Ft void *
 .Fn aligned_alloc "size_t alignment" "size_t size"
 .Ft size_t
-.Fn malloc_usable_size "void *ptr"
 .Vt char *malloc_options ;
 .Sh DESCRIPTION
 The standard functions
@@ -236,25 +234,6 @@ If
 is not a multiple of
 .Fa alignment ,
 behavior is undefined.
-.Pp
-The
-.Fn malloc_usable_size
-function returns the actual size of the allocated memory pointed to by
-.Va ptr .
-If
-.Va ptr
-is
-.Dv NULL ,
-it returns 0.
-If
-.Va ptr
-was never returned by an allocation function or freed before,
-the behavior is undefined.
-This function should not be relied upon since it exposes some of the internal
-workings of the
-.Fn malloc
-family of functions.
-Writing beyond the requested size introduces undefined behavior.
 .Sh RETURN VALUES
 Upon successful completion, the allocation functions
 return a pointer to the allocated space; otherwise,
@@ -640,9 +619,7 @@ function appeared in
 .Ox 6.2 .
 The
 .Fn aligned_alloc
-and
-.Fn malloc_usable_size
-functions appeared in
+function appeared in
 .Ox 6.5 .
 .Sh CAVEATS
 When using
diff --git a/src/lib/libc/stdlib/malloc.c b/src/lib/libc/stdlib/malloc.c
index 513defcc..0912b904 100644
--- a/src/lib/libc/stdlib/malloc.c
+++ b/src/lib/libc/stdlib/malloc.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: malloc.c,v 1.253 2018/11/19 22:50:24 guenther Exp $	*/
+/*	$OpenBSD: malloc.c,v 1.254 2018/11/21 06:57:04 otto Exp $	*/
 /*
  * Copyright (c) 2008, 2010, 2011, 2016 Otto Moerbeek <otto@drijf.net>
  * Copyright (c) 2012 Matthew Dempsky <matthew@openbsd.org>
@@ -1466,83 +1466,6 @@ freezero(void *ptr, size_t sz)
 }
 DEF_WEAK(freezero);
 
-static size_t
-osize(struct dir_info *argpool, void *p)
-{
-	struct dir_info *pool;
-	struct region_info *r;
-	char *saved_function;
-	size_t sz;
-	int i;
-
-	pool = argpool;
-	r = find(pool, p);
-	if (r == NULL) {
-		if (mopts.malloc_mt)  {
-			for (i = 0; i < _MALLOC_MUTEXES; i++) {
-				if (i == argpool->mutex)
-					continue;
-				pool->active--;
-				_MALLOC_UNLOCK(pool->mutex);
-				pool = mopts.malloc_pool[i];
-				_MALLOC_LOCK(pool->mutex);
-				pool->active++;
-				r = find(pool, p);
-				if (r != NULL) {
-					saved_function = pool->func;
-					pool->func = argpool->func;
-					break;
-				}
-			}
-		}
-		if (r == NULL)
-			wrterror(argpool, "bogus pointer (double free?) %p", p);
-	}
-
-	REALSIZE(sz, r);
-	if (sz > MALLOC_MAXCHUNK) {
-		if (MALLOC_MOVE_COND(sz))
-			sz = MALLOC_PAGESIZE - ((char *)p - (char *)r->p);
-		else
-			sz = PAGEROUND(sz);
-	}
-	if (argpool != pool) {
-		pool->active--;
-		pool->func = saved_function;
-		_MALLOC_UNLOCK(pool->mutex);
-		_MALLOC_LOCK(argpool->mutex);
-		argpool->active++;
-	}
-	return sz;
-}
-
-size_t
-malloc_usable_size(void *ptr)
-{
-	struct dir_info *d;
-	int saved_errno = errno;
-	size_t sz;
-
-	/* This is legal. */
-	if (ptr == NULL)
-		return 0;
-
-	d = getpool();
-	if (d == NULL)
-		wrterror(d, "malloc_usable_size() called before allocation");
-	_MALLOC_LOCK(d->mutex);
-	d->func = "malloc_usable_size";
-	if (d->active++) {
-		malloc_recurse(d);
-		return 0;
-	}
-	sz = osize(d, ptr);
-	d->active--;
-	_MALLOC_UNLOCK(d->mutex);
-	errno = saved_errno;
-	return sz;
-}
-
 static void *
 orealloc(struct dir_info *argpool, void *p, size_t newsz, void *f)
 {