diff --git a/share/man/man5/tmpfs.5 b/share/man/man5/tmpfs.5
--- a/share/man/man5/tmpfs.5
+++ b/share/man/man5/tmpfs.5
@@ -164,6 +164,24 @@
 Refer to
 .Xr mount 8 .
 .El
+.Sh SYSCTL VARIABLES
+The following
+.Xr sysctl 8
+variables are available:
+.Bl -tag -width indent
+.It Va vfs.tmpfs.memory_percent
+The percentage of memory plus swap space available at kernel file system
+initialization that can be used by a file system with a size of 0.
+When this amount of space in use is reached, new files cannot be created
+and files cannot be extended.
+The default is 95%.
+Changing this value also changes
+.Va vfs.tmpfs.memory_reserved .
+.It Va vfs.tmpfs.memory_reserved
+The currently-reserved amount of memory plus swap space
+based on the memory percentage.
+The minimum is compiled into the system, and defaults to 4 MB.
+.El
 .Sh EXAMPLES
 Mount a
 .Nm
diff --git a/sys/fs/tmpfs/tmpfs.h b/sys/fs/tmpfs/tmpfs.h
--- a/sys/fs/tmpfs/tmpfs.h
+++ b/sys/fs/tmpfs/tmpfs.h
@@ -543,6 +543,14 @@
 #define TMPFS_PAGES_MINRESERVED		(4 * 1024 * 1024 / PAGE_SIZE)
 #endif
 
+/*
+ * Percent of available memory + swap available to use by tmpfs file systems
+ * without a size limit.
+ */
+#if !defined(TMPFS_MEM_PERCENT)
+#define TMPFS_MEM_PERCENT		95
+#endif
+
 /*
  * Amount of memory to reserve for extended attributes.
  */
diff --git a/sys/fs/tmpfs/tmpfs_subr.c b/sys/fs/tmpfs/tmpfs_subr.c
--- a/sys/fs/tmpfs/tmpfs_subr.c
+++ b/sys/fs/tmpfs/tmpfs_subr.c
@@ -73,6 +73,9 @@
     "tmpfs file system");
 
 static long tmpfs_pages_reserved = TMPFS_PAGES_MINRESERVED;
+static long tmpfs_pages_avail_init;
+static int tmpfs_mem_percent = TMPFS_MEM_PERCENT;
+static void tmpfs_set_reserve_from_percent(void);
 
 MALLOC_DEFINE(M_TMPFSDIR, "tmpfs dir", "tmpfs dirent structure");
 static uma_zone_t tmpfs_node_pool;
@@ -367,6 +370,9 @@
 	    sizeof(struct tmpfs_node), tmpfs_node_ctor, tmpfs_node_dtor,
 	    tmpfs_node_init, tmpfs_node_fini, UMA_ALIGN_PTR, 0);
 	VFS_SMR_ZONE_SET(tmpfs_node_pool);
+
+	tmpfs_pages_avail_init = tmpfs_mem_avail();
+	tmpfs_set_reserve_from_percent();
 	return (0);
 }
 
@@ -401,10 +407,42 @@
 }
 
 SYSCTL_PROC(_vfs_tmpfs, OID_AUTO, memory_reserved,
-    CTLTYPE_LONG|CTLFLAG_MPSAFE|CTLFLAG_RW, &tmpfs_pages_reserved, 0,
+    CTLTYPE_LONG | CTLFLAG_MPSAFE | CTLFLAG_RW, &tmpfs_pages_reserved, 0,
     sysctl_mem_reserved, "L",
     "Amount of available memory and swap below which tmpfs growth stops");
 
+static int
+sysctl_mem_percent(SYSCTL_HANDLER_ARGS)
+{
+	int error, percent;
+
+	percent = *(int *)arg1;
+	error = sysctl_handle_int(oidp, &percent, 0, req);
+	if (error || !req->newptr)
+		return (error);
+
+	if ((unsigned) percent > 100)
+		return (EINVAL);
+
+	*(long *)arg1 = percent;
+	tmpfs_set_reserve_from_percent();
+	return (0);
+}
+
+static void
+tmpfs_set_reserve_from_percent(void)
+{
+	size_t reserved;
+
+	reserved = tmpfs_pages_avail_init * (100 - tmpfs_mem_percent) / 100;
+	tmpfs_pages_reserved = max(reserved, TMPFS_PAGES_MINRESERVED);
+}
+
+SYSCTL_PROC(_vfs_tmpfs, OID_AUTO, memory_percent,
+    CTLTYPE_INT | CTLFLAG_MPSAFE | CTLFLAG_RW, &tmpfs_mem_percent, 0,
+    sysctl_mem_percent, "I",
+    "Percent of available memory that can be used if no size limit");
+
 static __inline int tmpfs_dirtree_cmp(struct tmpfs_dirent *a,
     struct tmpfs_dirent *b);
 RB_PROTOTYPE_STATIC(tmpfs_dir, tmpfs_dirent, uh.td_entries, tmpfs_dirtree_cmp);