NFS: Add the mount option "nosharecache"

Prior to David Howell's mount changes in 2.6.18, users who mounted
different directories which happened to be from the same filesystem on the
server would get different super blocks, and hence could choose different
mount options. As long as there were no hard linked files that crossed from
one subtree to another, this was quite safe.
Post the changes, if the two directories are on the same filesystem (have
the same 'fsid'), they will share the same super block, and hence the same
mount options.

Add a flag to allow users to elect not to share the NFS super block with
another mount point, even if the fsids are the same. This will allow
users to set different mount options for the two different super blocks, as
was previously possible. It is still up to the user to ensure that there
are no cache coherency issues when doing this, however the default
behaviour will be to share super blocks whenever two paths result in
the same fsid.

Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/super.c

···
 	Opt_udp, Opt_tcp,
 	Opt_acl, Opt_noacl,
 	Opt_rdirplus, Opt_nordirplus,
+	Opt_sharecache, Opt_nosharecache,
 
 	/* Mount options that take integer arguments */
 	Opt_port,
···
 	{ Opt_noacl, "noacl" },
 	{ Opt_rdirplus, "rdirplus" },
 	{ Opt_nordirplus, "nordirplus" },
+	{ Opt_sharecache, "sharecache" },
+	{ Opt_nosharecache, "nosharecache" },
 
 	{ Opt_port, "port=%u" },
 	{ Opt_rsize, "rsize=%u" },
···
 		{ NFS_MOUNT_NONLM, ",nolock", "" },
 		{ NFS_MOUNT_NOACL, ",noacl", "" },
 		{ NFS_MOUNT_NORDIRPLUS, ",nordirplus", "" },
+		{ NFS_MOUNT_UNSHARED, ",nosharecache", ""},
 		{ 0, NULL, NULL }
 	};
 	const struct proc_nfs_info *nfs_infop;
···
 			break;
 		case Opt_nordirplus:
 			mnt->flags |= NFS_MOUNT_NORDIRPLUS;
+			break;
+		case Opt_sharecache:
+			mnt->flags &= ~NFS_MOUNT_UNSHARED;
+			break;
+		case Opt_nosharecache:
+			mnt->flags |= NFS_MOUNT_UNSHARED;
 			break;
 
 		case Opt_port:
···
 
 	if (old->nfs_client != server->nfs_client)
 		return 0;
+	/* Note: NFS_MOUNT_UNSHARED == NFS4_MOUNT_UNSHARED */
+	if (old->flags & NFS_MOUNT_UNSHARED)
+		return 0;
 	if (memcmp(&old->fsid, &server->fsid, sizeof(old->fsid)) != 0)
 		return 0;
 	return 1;
···
 	struct nfs_fh mntfh;
 	struct nfs_mount_data *data = raw_data;
 	struct dentry *mntroot;
+	int (*compare_super)(struct super_block *, void *) = nfs_compare_super;
 	int error;
 
 	/* Validate the mount data */
···
 		goto out;
 	}
 
+	if (server->flags & NFS_MOUNT_UNSHARED)
+		compare_super = NULL;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
-	s = sget(fs_type, nfs_compare_super, nfs_set_super, server);
+	s = sget(fs_type, compare_super, nfs_set_super, server);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto out_err_nosb;
···
 	struct super_block *s;
 	struct nfs_server *server;
 	struct dentry *mntroot;
+	int (*compare_super)(struct super_block *, void *) = nfs_compare_super;
 	int error;
 
 	dprintk("--> nfs_xdev_get_sb()\n");
···
 		goto out_err_noserver;
 	}
 
+	if (server->flags & NFS_MOUNT_UNSHARED)
+		compare_super = NULL;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
-	s = sget(&nfs_fs_type, nfs_compare_super, nfs_set_super, server);
+	s = sget(&nfs_fs_type, compare_super, nfs_set_super, server);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto out_err_nosb;
···
 	struct nfs_fh mntfh;
 	struct dentry *mntroot;
 	char *mntpath = NULL, *hostname = NULL, *ip_addr = NULL;
+	int (*compare_super)(struct super_block *, void *) = nfs_compare_super;
 	int error;
 
 	/* Validate the mount data */
···
 		goto out;
 	}
 
+	if (server->flags & NFS4_MOUNT_UNSHARED)
+		compare_super = NULL;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
-	s = sget(fs_type, nfs_compare_super, nfs_set_super, server);
+	s = sget(fs_type, compare_super, nfs_set_super, server);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto out_free;
···
 	struct super_block *s;
 	struct nfs_server *server;
 	struct dentry *mntroot;
+	int (*compare_super)(struct super_block *, void *) = nfs_compare_super;
 	int error;
 
 	dprintk("--> nfs4_xdev_get_sb()\n");
···
 		goto out_err_noserver;
 	}
 
+	if (server->flags & NFS4_MOUNT_UNSHARED)
+		compare_super = NULL;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
-	s = sget(&nfs_fs_type, nfs_compare_super, nfs_set_super, server);
+	s = sget(&nfs_fs_type, compare_super, nfs_set_super, server);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto out_err_nosb;
···
 	struct nfs_server *server;
 	struct dentry *mntroot;
 	struct nfs_fh mntfh;
+	int (*compare_super)(struct super_block *, void *) = nfs_compare_super;
 	int error;
 
 	dprintk("--> nfs4_referral_get_sb()\n");
···
 		goto out_err_noserver;
 	}
 
+	if (server->flags & NFS4_MOUNT_UNSHARED)
+		compare_super = NULL;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
-	s = sget(&nfs_fs_type, nfs_compare_super, nfs_set_super, server);
+	s = sget(&nfs_fs_type, compare_super, nfs_set_super, server);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto out_err_nosb;

include/linux/nfs4_mount.h

···
 #define NFS4_MOUNT_NOCTO	0x0010	/* 1 */
 #define NFS4_MOUNT_NOAC		0x0020	/* 1 */
 #define NFS4_MOUNT_STRICTLOCK	0x1000	/* 1 */
-#define NFS4_MOUNT_FLAGMASK	0x1033
+#define NFS4_MOUNT_UNSHARED	0x8000	/* 1 */
+#define NFS4_MOUNT_FLAGMASK	0x9033
 
 #endif

include/linux/nfs_mount.h

···
 #define NFS_MOUNT_STRICTLOCK	0x1000	/* reserved for NFSv4 */
 #define NFS_MOUNT_SECFLAVOUR	0x2000	/* 5 */
 #define NFS_MOUNT_NORDIRPLUS	0x4000	/* 5 */
+#define NFS_MOUNT_UNSHARED	0x8000	/* 5 */
 #define NFS_MOUNT_FLAGMASK	0xFFFF
 
 #endif