linux-kernel - Re: [PATCH 2/2] ovl: allow distributed fs as lower layer

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Message-ID: <20150605150939.GA2174@tucsk>
Date:	Fri, 5 Jun 2015 17:37:29 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, sa-dev@...nbow.by,
	andre.roth@...he.com
Subject: Re: [PATCH 2/2] ovl: allow distributed fs as lower layer

On Fri, Jun 05, 2015 at 01:07:15AM +0100, Al Viro wrote:

> Umm...  Cosmetical point is that this
> 
> > +static bool ovl_remote(struct dentry *root)
> > +{
> > +	const struct dentry_operations *dop = root->d_op;
> > +
> > +	return dop && (dop->d_revalidate || dop->d_weak_revalidate);
> > +}
> 
> is better done as
> 	root->d_flags & (DCACHE_OP_REVALIDATE | DCACHE_OP_WEAK_REVALIDATE)

Okay.

> 
> More interesting question is whether anything in the system relies on
> existing behaviour that follows ->d_revalidate() returning 0.

Hmm, d_invalidate() almost always follows ->d_revalidate().  Almost, becuase RCU
lookup can get aborted at that point.  We can easily stick d_invalidate() in
there for the non-RCU case.

Regular lookup also almost always follows ->d_revalidate().  Except if
allocation of new dentry fails.  So relying on this would be buggy (which is not
to say nobody does it).

>  Have you tried to mount e.g. procfs as underlying layer and torture it for a
> while?

I did try now.  Nothing bad happened during the test (parallel stat(1) of the
whole overlayed proc tree).

My laptop froze while trying to write this mail.  But it's 8 years old and when
the fan starts to make noises and the weather is hot, it does this sometimes.  I
don't think that has anything to do with overlayfs, but will do more testing...

Thanks,
Miklos
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