From 99f390a4e8015ddd72e91fd20840ca3847704ec6 Mon Sep 17 00:00:00 2001
From: rmacklem <rmacklem@FreeBSD.org>
Date: Tue, 13 Sep 2011 21:01:26 +0000
Subject: [PATCH] Modify vfs_register() to use a hash calculation on vfc_name
 to set vfc_typenum, so that vfc_typenum doesn't change when file systems are
 loaded in different orders. This keeps NFS file handles from changing, for
 file systems that use vfc_typenum in their fsid. This change is controlled
 via a loader.conf variable called vfs.typenumhash, since vfc_typenum will
 change once when this is enabled. It defaults to 1 for 9.0, but will default
 to 0 when MFC'd to stable/8.

Tested by:	hrs
Reviewed by:	jhb, pjd (earlier version)
Approved by:	re (kib)
MFC after:	1 month
---
 UPDATING            | 12 ++++++++++++
 sys/kern/vfs_init.c | 45 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 56 insertions(+), 1 deletion(-)

diff --git a/UPDATING b/UPDATING
index f5c6f4de7ad..dd351316650 100644
--- a/UPDATING
+++ b/UPDATING
@@ -22,6 +22,18 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 9.x IS SLOW:
 	machines to maximize performance.  (To disable malloc debugging, run
 	ln -s aj /etc/malloc.conf.)
 
+20110913:
+	This commit modifies vfs_register() so that it uses a hash
+	calculation to set vfc_typenum, which is enabled by default.
+	The first time a system is booted after this change, the
+	vfc_typenum values will change for all file systems. The
+	main effect of this is a change to the NFS server file handles
+	for file systems that use vfc_typenum in their fsid, such as ZFS.
+	It will, however, prevent vfc_typenum from changing when file
+	systems are loaded in a different order for subsequent reboots.
+	To disable this, you can set vfs.typenumhash=0 in /boot/loader.conf
+	until you are ready to remount all NFS clients after a reboot.
+
 20110828:
 	Bump the shared library version numbers for libraries that
 	do not use symbol versioning, have changed the ABI compared
diff --git a/sys/kern/vfs_init.c b/sys/kern/vfs_init.c
index 4367b8c8096..0165b38c3cd 100644
--- a/sys/kern/vfs_init.c
+++ b/sys/kern/vfs_init.c
@@ -39,6 +39,7 @@ __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/fnv_hash.h>
 #include <sys/kernel.h>
 #include <sys/linker.h>
 #include <sys/mount.h>
@@ -64,6 +65,18 @@ int maxvfsconf = VFS_GENERIC + 1;
  */
 struct vfsconfhead vfsconf = TAILQ_HEAD_INITIALIZER(vfsconf);
 
+/*
+ * Loader.conf variable vfs.typenumhash enables setting vfc_typenum using a hash
+ * calculation on vfc_name, so that it doesn't change when file systems are
+ * loaded in a different order. This will avoid the NFS server file handles from
+ * changing for file systems that use vfc_typenum in their fsid.
+ */
+static int	vfs_typenumhash = 1;
+TUNABLE_INT("vfs.typenumhash", &vfs_typenumhash);
+SYSCTL_INT(_vfs, OID_AUTO, typenumhash, CTLFLAG_RDTUN, &vfs_typenumhash, 0,
+    "Set vfc_typenum using a hash calculation on vfc_name, so that it does not"
+    "change when file systems are loaded in a different order.");
+
 /*
  * A Zen vnode attribute structure.
  *
@@ -138,6 +151,9 @@ vfs_register(struct vfsconf *vfc)
 	struct sysctl_oid *oidp;
 	struct vfsops *vfsops;
 	static int once;
+	struct vfsconf *tvfc;
+	uint32_t hashval;
+	int secondpass;
 
 	if (!once) {
 		vattr_null(&va_null);
@@ -152,7 +168,34 @@ vfs_register(struct vfsconf *vfc)
 	if (vfs_byname(vfc->vfc_name) != NULL)
 		return EEXIST;
 
-	vfc->vfc_typenum = maxvfsconf++;
+	if (vfs_typenumhash != 0) {
+		/*
+		 * Calculate a hash on vfc_name to use for vfc_typenum. Unless
+		 * all of 1<->255 are assigned, it is limited to 8bits since
+		 * that is what ZFS uses from vfc_typenum and is also the
+		 * preferred range for vfs_getnewfsid().
+		 */
+		hashval = fnv_32_str(vfc->vfc_name, FNV1_32_INIT);
+		hashval &= 0xff;
+		secondpass = 0;
+		do {
+			/* Look for and fix any collision. */
+			TAILQ_FOREACH(tvfc, &vfsconf, vfc_list) {
+				if (hashval == tvfc->vfc_typenum) {
+					if (hashval == 255 && secondpass == 0) {
+						hashval = 1;
+						secondpass = 1;
+					} else
+						hashval++;
+					break;
+				}
+			}
+		} while (tvfc != NULL);
+		vfc->vfc_typenum = hashval;
+		if (vfc->vfc_typenum >= maxvfsconf)
+			maxvfsconf = vfc->vfc_typenum + 1;
+	} else
+		vfc->vfc_typenum = maxvfsconf++;
 	TAILQ_INSERT_TAIL(&vfsconf, vfc, vfc_list);
 
 	/*
-- 
2.45.0