Re: NFSD tapset: how can I get filename from filehandles ?

[David Smith <dsmith at redhat dot com>]

On 08/24/2010 09:23 AM, sbocahu@bearstech.com wrote:
> Hi all,
> 
> I hope I'm not disturbing too much... I'm a new systemtap user without any
> kernel knowledge, trying to write a few scripts.
> Since I fail to write the second one, I would like to ask you for
> pointers:

No problem - ask away.

> I would like to gather some stats from nfsd probes to know which files are
> most accessed/written/read/etc...
> This is pretty easy with the create(), remove() and lookup() nfsops as the
> nfsd tapset exposes the filename variable (wich seems to be easy as well,
> because the filename is given as argument to the call).
> 
> The problem is: I can't get the filename for other nfsops. The filename is
> not given as argument.

Right.

> I guess it is possible to get it from the filehandle, though:
> (warning: I might be _totally_ wrong, I just tried to read the source and
> understand things by myself...)

... stuff deleted ...

> I've tried a few things without success... such as:
> 
> 
>     filename = @cast($argp, "nfsd3_writeargs",
> "kernel:nfsd")->fh->fh_dentry->d_name->name
> 
> 
> which ended with a segfault.

OK, I'm going to need some details here.  First, what kernel are you using?

When you say a 'segfault', do you mean the stap executable segfaulted,
or you got a kernel read fault, something like this:

---
ERROR: kernel read fault at 0x0000000000000034 (addr) near identifier
'@cast' at /usr/local/share/systemtap/tapset/dentry.stp:41:15
---

I'll include the script in a sec, but that is systemtap protecting me
from crashing my kernel.  Instead of letting me read from that bogus
address, systemtap errors out.  This is a good thing.

If the stap executable segfaulted, could you show us your script and
exact output you are seeing?

> Well, would you correct me and give me pointers on how achieving this,
> please ?

I need to do something like this myself, so I've looked into it.  Here's
what I started with:

==========
probe nfsd.proc3.write
{
	printf("fh = %p\n", fh)
	dentry = @cast(fh, "svc_fh", "kernel:nfsd")->fh_dentry
	printf("dentry = %p\n", dentry)
	printf("writing %s\n", d_name(dentry))
}
==========

That's what gives the kernel read fault I showed above.  Here's a
"fixed" version - it doesn't error out, but it doesn't work either.

==========
probe nfsd.proc3.write
{
	printf("fh = %p\n", fh)
	dentry = @cast(fh, "svc_fh", "kernel:nfsd")->fh_dentry
	printf("dentry = %p\n", dentry)
	if (dentry)
		printf("writing %s\n", d_name(dentry))
	else
		printf("no dentry!\n")
}
==========

At that point dentry is NULL.

When(/if) I figure out how to get the filename, I'll let you know.  If
you figure it out, let me know.

Feel free to email the list with more questions.

-- 
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)