If a client mounts a subdirectory of an export using NFSv3, the server can end
up with invalid (CACHE_VALID bit unset) entries in its export cache. These
entries indirectly have a reference to the struct mount* containing the export,
preventing the filesystem containing the export from being unmounted, even
after the export has been unexported:
/tmp# mount --bind foo bar
/tmp# exportfs -o fsid=7 '*:/tmp/bar'
/tmp# mount -t nfs -o vers=3 localhost:/tmp/bar/a /mnt/a
/tmp# umount /mnt/a
/tmp# exportfs -u '*:/tmp/bar'
/tmp# umount /tmp/bar
umount: /tmp/bar: target is busy.
/tmp# cat /proc/net/rpc/nfsd.export/content
#path domain(flags)
# /tmp/bar/a *()
It looks like what's happening is that when rpc.mountd does a downcall to get a
filehandle corresponding to a particular path, exp_parent() traverses the
elements in the given path looking for one which is in the export cache. If
there isn't a valid entry in the cache for the given path, which there won't be
for a subdirectory of an export, then sunrpc_cache_lookup_rcu() inserts an
new, invalid entry.
Are the invalid entries inserted while executing exp_parent() useful? Could we
prevent them from being added?
Thanks,
John
