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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jun 04, 2015 at 03:29:46PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Allow filesystems with .d_revalidate as lower layer(s), but not as upper
> layer.
> 
> For local filesystems the rule was that modifications on the layers
> directly while being part of the overlay results in undefined behavior.
> 
> This can easily be extended to distributed filesystems: we assume the tree
> used as lower layer is static, which means ->d_revalidate() should always
> return "1".  If that is not the case, return -ESTALE, don't try to work
> around the modification.

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)

More interesting question is whether anything in the system relies on
existing behaviour that follows ->d_revalidate() returning 0.  Have you
tried to mount e.g. procfs as underlying layer and torture it for a while?