On Mon, 7 Mar 2016, Al Viro wrote:
> On Wed, Mar 02, 2016 at 11:00:01AM +0800, Yan, Zheng wrote:
>
> > > This code dates back to when Ceph was originally upstreamed, so the
> > > history is murky, but I expect at that point I wanted to avoid hashing in
> > > the no-lease case. But I don't think it matters. We should just remove
> > > the prehash argument from splice_dentry entirely.
> > >
> > > Zheng, does that sound right?
> >
> > Yes. I think we can remove the d_rehash(dn) call and rehash parameter.
>
> Another question in the same general area:
> /* null dentry? */
> if (!rinfo->head->is_target) {
> dout("fill_trace null dentry\n");
> if (d_really_is_positive(dn)) {
> ceph_dir_clear_ordered(dir);
> dout("d_delete %p\n", dn);
> d_delete(dn);
> } else {
> dout("d_instantiate %p NULL\n", dn);
> d_instantiate(dn, NULL);
> if (have_lease && d_unhashed(dn))
> d_rehash(dn);
> update_dentry_lease(dn, rinfo->dlease,
> session,
> req->r_request_started);
> }
> goto done;
> }
> What's that d_instantiate() about? We have just checked that it's
> negative; what's the point of setting ->d_inode to NULL again? Would it
> be OK if we just do
> } else {
> if (have_lease && d_unhashed(dn))
> d_add(dn, NULL);
> update_dentry_lease(dn, rinfo->dlease,
> session,
> req->r_request_started);
> }
> in there?
That looks okay, but changing d_rehash to d_add still means you're doing
te d_instantiate(dn, NULL) in the d_unhashed case; is there a reason you
changed that line? Is the dentry_rcuwalk_invalidate in __d_instantiate is
important before rehashing?
> As an aside, tracking back to the originating fs method is
> painful as hell ;-/ I _think_ that rehash can be hit during ->lookup()
> returning a negative, but I wouldn't bet a dime on it not happening from
> other methods... AFAICS, the change should be OK regardless of what
> it's been called from, but... _ouch_. Is is documented anywhere public?
It is a pain to follow, yes. FWIW this whole block is predicated in
req->r_locked_dir being non-NULL (i.e., VFS holds dir->i_mutex), which is
only true for lookup, create operations (mkdir/mknod/symlink/etc.),
atomic_open, and the .get_name export op. There's not much documentation
beyond a description of the meaning of fields (e.g. r_locked_dir) in
fs/ceph/mds_client.h ...
sage
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