what's the real meaning of fsid?

["lioupayphone" <lioupayphone@gmail.com>]

Hello, everyone. 

fsid in /etc/exports was used for identifying a file system. if a file system which exported 2 directories, it seems that we should tag the two export entries with same fsid.

eg
on one machine (server   10.10.37.147,  Centos5.2 with linux2.6.18):

#mkfs.ext3 /dev/sdc; mount /dev/sdc /mnt/;mkdir -p /mnt/dir1 /mnt/dir2; touch /mnt/dir1/wall-e /mnt/dir2/eva;
#echo "/mnt/dir1 *(rw,async,root_squash,fsid=2)" > /etc/exports
#echo "/mnt/dir2 *(rw,async,root_squash,fsid=2)" >> /etc/exports
#service nfs start && exportfs -r

on another machine (client   10.10.37.154):
#mount  10.10.37.147:/mnt/dir1 /mnt/1/ && mount 10.10.37.147:/mnt/dir2 /mnt/2/

i am puzzled:   on the client (10.10.37.154), i found both "/mnt/1/" and "/mnt/2/" have the same child ---- "wall-e".

so i browsed the code of 2.6.18 and found: exp_export(), fs/nfsd/export.c . i have taged a comment on this code listed below. please give me some suggestions. thx.

/*
 * Export a file system.
 */
int
exp_export(struct nfsctl_export *nxp)
{
	svc_client	*clp;
	struct svc_export	*exp = NULL;
	struct svc_export	new;
	struct svc_expkey	*fsid_key = NULL;
	struct nameidata nd;
	int		err;

	/* Consistency check */
	err = -EINVAL;
	if (!exp_verify_string(nxp->ex_path, NFS_MAXPATHLEN) ||
	    !exp_verify_string(nxp->ex_client, NFSCLNT_IDMAX))
		goto out;

	dprintk("exp_export called for %s:%s (%x/%ld fl %x).\n",
			nxp->ex_client, nxp->ex_path,
			(unsigned)nxp->ex_dev, (long)nxp->ex_ino,
			nxp->ex_flags);

	/* Try to lock the export table for update */
	exp_writelock();

	/* Look up client info */
	if (!(clp = auth_domain_find(nxp->ex_client)))
		goto out_unlock;


	/* Look up the dentry */
	err = path_lookup(nxp->ex_path, 0, &nd);
	if (err)
		goto out_unlock;
	err = -EINVAL;

	exp = exp_get_by_name(clp, nd.mnt, nd.dentry, NULL);

	/* must make sure there won't be an ex_fsid clash */
	if ((nxp->ex_flags & NFSEXP_FSID) &&
	    (fsid_key = exp_get_fsid_key(clp, nxp->ex_dev)) &&
	    !IS_ERR(fsid_key) &&
	    fsid_key->ek_mnt &&
		/* 
		 * --> HERE.  "nd.mnt of "/mnt/dir1" and "/mnt/dir2" are same, but nd.dentry not.
		 * "(fsid_key->ek_mnt != nd.mnt || fsid_key->ek_dentry != nd.dentry)" will be 1 in such
		 * a situation. but they are different exp-entries in "/var/lib/nfs/etab"
		* it should be "(fsid_key->ek_dentry != nd.dentry || fsid_key->ek_mnt != nd.mnt)" ??
		 */
	    (fsid_key->ek_mnt != nd.mnt || fsid_key->ek_dentry != nd.dentry) )        
		goto finish;

	if (exp) {
		/* just a flags/id/fsid update */

		exp_fsid_unhash(exp);
		exp->ex_flags    = nxp->ex_flags;
		exp->ex_anon_uid = nxp->ex_anon_uid;
		exp->ex_anon_gid = nxp->ex_anon_gid;
		exp->ex_fsid     = nxp->ex_dev;

		err = exp_fsid_hash(clp, exp);
		goto finish;
	}

	err = check_export(nd.dentry->d_inode, nxp->ex_flags);
	if (err) goto finish;

	err = -ENOMEM;

	dprintk("nfsd: creating export entry %p for client %p\n", exp, clp);

	new.h.expiry_time = NEVER;
	new.h.flags = 0;
	new.ex_client = clp;
	new.ex_mnt = nd.mnt;
	new.ex_dentry = nd.dentry;
	new.ex_flags = nxp->ex_flags;
	new.ex_anon_uid = nxp->ex_anon_uid;
	new.ex_anon_gid = nxp->ex_anon_gid;
	new.ex_fsid = nxp->ex_dev;

	exp = svc_export_lookup(&new);
	if (exp)
		exp = svc_export_update(&new, exp);

	if (!exp)
		goto finish;

	if (exp_hash(clp, exp) ||
	    exp_fsid_hash(clp, exp)) {
		/* failed to create at least one index */
		exp_do_unexport(exp);
		cache_flush();
		err = -ENOMEM;
	}

finish:
	if (exp)
		exp_put(exp);
	if (fsid_key && !IS_ERR(fsid_key))
		cache_put(&fsid_key->h, &svc_expkey_cache);
	if (clp)
		auth_domain_put(clp);
	path_release(&nd);
out_unlock:
	exp_writeunlock();
out:
	return err;
}
	
	
 				
Best Regards! 
lioupayphone