"nosharecache" option prevent "mount" from detecting when the mount is a duplicate.

[NeilBrown <neilb@suse.de>]

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(I seem to have quite a pile of NFS issues lately.... some if it is tidying up
issues from before Christmas, some of it just keeps on coming :-)

If you run "mount -a" it will attempt to mount all filesystems listed
in /etc/fstab, but filesystems that are already mounted will not be mounted
again.  So it is normally safe to run "mount -a" multiple times.

However if an NFS mount in /etc/fstab has the "nosharecache" option set,
mount doesn't notice that it is already mounted as so mounts it again.  So
repeated "mount -a" is no longer safe.

This happens because the prevention of multiple mounts happens in do_add_mount
in fs/namespace.c:

	err = -EBUSY;
	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
	    path->mnt->mnt_root == path->dentry)
		goto unlock;


i.e. if the exact same superblock is being mounted in the exact same place,
return EBUSY.
With nosharecache, every mount attempt produces a new superblock so this test
never fires.

One way to address this would be to have a different option, e.g.
          sharecache=27

where the '27' is an arbitrary number meaning that if two mount attempts have
different sharecache numbers they will have different superblocks.  If they
have the same sharecache number they can have the same superblock.
This is not the most elegant interface ever and I would be very happy for
suggestions to improve it.  Maybe a string rather than a number ???

This probably isn't a very serious issue, but is a regression in terms of the
usability of "mount -a" and I think it would be best to fix it if the cost is
not too high.

Below is my patch to implement the "sharecache=%u" syntax.

Any ideas?

Thanks,
NeilBrown

diff --git a/fs/nfs/client.c b/fs/nfs/client.c
index 1d09289c8f0e..7ed2f1e37a18 100644
--- a/fs/nfs/client.c
+++ b/fs/nfs/client.c
@@ -779,6 +779,7 @@ static int nfs_init_server(struct nfs_server *server,
 	server->acregmax = data->acregmax * HZ;
 	server->acdirmin = data->acdirmin * HZ;
 	server->acdirmax = data->acdirmax * HZ;
+	server->cache_id = data->cache_id;
 
 	/* Start lockd here, before we might error out */
 	error = nfs_start_lockd(server);
@@ -931,6 +932,7 @@ void nfs_server_copy_userdata(struct nfs_server *target, struct nfs_server *sour
 	target->caps = source->caps;
 	target->options = source->options;
 	target->auth_info = source->auth_info;
+	target->cache_id = source->cache_id;
 }
 EXPORT_SYMBOL_GPL(nfs_server_copy_userdata);
 
diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 8b5cc04a8611..b1c2b9164f25 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -95,6 +95,7 @@ struct nfs_parsed_mount_data {
 	unsigned int		minorversion;
 	char			*fscache_uniq;
 	bool			need_mount;
+	unsigned int		cache_id;
 
 	struct {
 		struct sockaddr_storage	address;
diff --git a/fs/nfs/nfs4client.c b/fs/nfs/nfs4client.c
index dbb3e1f30c68..25f426db68f9 100644
--- a/fs/nfs/nfs4client.c
+++ b/fs/nfs/nfs4client.c
@@ -997,6 +997,7 @@ static int nfs4_init_server(struct nfs_server *server,
 	server->acregmax = data->acregmax * HZ;
 	server->acdirmin = data->acdirmin * HZ;
 	server->acdirmax = data->acdirmax * HZ;
+	server->cache_id = data->cache_id;
 
 	server->port = data->nfs_server.port;
 
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index 910ed906eb82..27edb581affa 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -103,6 +103,7 @@ enum {
 	Opt_mountport,
 	Opt_mountvers,
 	Opt_minorversion,
+	Opt_sharecacheid,
 
 	/* Mount options that take string arguments */
 	Opt_nfsvers,
@@ -145,6 +146,7 @@ static const match_table_t nfs_mount_option_tokens = {
 	{ Opt_rdirplus, "rdirplus" },
 	{ Opt_nordirplus, "nordirplus" },
 	{ Opt_sharecache, "sharecache" },
+	{ Opt_sharecacheid, "sharecache=%s" },
 	{ Opt_nosharecache, "nosharecache" },
 	{ Opt_resvport, "resvport" },
 	{ Opt_noresvport, "noresvport" },
@@ -665,6 +667,10 @@ static void nfs_show_mount_options(struct seq_file *m, struct nfs_server *nfss,
 		else
 			seq_puts(m, nfs_infop->nostr);
 	}
+	if (!(nfss->flags & NFS_MOUNT_UNSHARED) &&
+	    nfss->cache_id)
+		seq_printf(m, ",sharecache=%u", nfss->cache_id);
+
 	rcu_read_lock();
 	seq_printf(m, ",proto=%s",
 		   rpc_peeraddr2str(nfss->client, RPC_DISPLAY_NETID));
@@ -1407,6 +1413,12 @@ static int nfs_parse_mount_options(char *raw,
 				goto out_invalid_value;
 			mnt->minorversion = option;
 			break;
+		case Opt_sharecacheid:
+			if (nfs_get_option_ul(args, &option) ||
+			    option <= 0)
+				goto out_invalid_value;
+			mnt->cache_id = option;
+			break;
 
 		/*
 		 * options that take text values
@@ -2457,6 +2469,9 @@ static int nfs_compare_super(struct super_block *sb, void *data)
 	/* Note: NFS_MOUNT_UNSHARED == NFS4_MOUNT_UNSHARED */
 	if (old->flags & NFS_MOUNT_UNSHARED)
 		return 0;
+	if ((old->cache_id || server->cache_id) &&
+	    old->cache_id != server->cache_id)
+		return 0;
 	if (memcmp(&old->fsid, &server->fsid, sizeof(old->fsid)) != 0)
 		return 0;
 	return nfs_compare_mount_options(sb, server, mntflags);
diff --git a/include/linux/nfs_fs_sb.h b/include/linux/nfs_fs_sb.h
index 1150ea41b626..8e66d95283e1 100644
--- a/include/linux/nfs_fs_sb.h
+++ b/include/linux/nfs_fs_sb.h
@@ -145,6 +145,7 @@ struct nfs_server {
 #define NFS_OPTION_FSCACHE	0x00000001	/* - local caching enabled */
 #define NFS_OPTION_MIGRATION	0x00000002	/* - NFSv4 migration enabled */
 
+	unsigned int		cache_id;	/* For NFS_MOUNT_UNSHARED */
 	struct nfs_fsid		fsid;
 	__u64			maxfilesize;	/* maximum file size */
 	struct timespec		time_delta;	/* smallest time granularity */

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