linux-kernel - Re: [ceph] what's going on with d_rehash() in splice

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Message-ID: <20160307021639.GR17997@ZenIV.linux.org.uk>
Date:	Mon, 7 Mar 2016 02:16:39 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Yan, Zheng" <zyan@...hat.com>
Cc:	Sage Weil <sweil@...hat.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, ceph-devel@...r.kernel.org
Subject: Re: [ceph] what's going on with d_rehash() in splice_dentry()?

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?  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?