linux-kernel - Re: [patch 4/8] fs, exportfs: Add export_encode_inode

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Date:	Thu, 16 Aug 2012 16:38:14 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	James Bottomley <jbottomley@...allels.com>,
	Matthew Helsley <matt.helsley@...il.com>
Subject: Re: [patch 4/8] fs, exportfs: Add export_encode_inode_fh helper

On Thu, Aug 16, 2012 at 10:24:48AM +0400, Cyrill Gorcunov wrote:
> > > > On the other hand, if you want a real filehandle then wouldn't you want
> > > > to e.g. call the filesystem's ->encode_fh() if necessary, as
> > > > exportfs_encode_fh() does?
> > > 
> > > Well, one of the problem I hit when I've been trying to use encode_fh
> > > is that every new implementation of encode_fh will require some size
> > > (even unknown) in buffer where encoded data pushed. Correct me please
> > > if I'm wrong. But with export_encode_inode_fh there is a small buffer
> > > with pretty known size needed on stack needed for printing data in
> > > fdinfo.
> > 
> > You can just give encode_fh a too-small data and let it fail if it's not
> > big enough.
> > 
> > (In practice I think everyone supports NFSv3 filehandles which have a
> > maximum size of 64 bytes.)
> 
> I'll think about it, thanks!

Hi Bruce, thinking a bit more I guess using general encode_fh is not that
convenient since it operates with dentries while our fdinfo output deals
with inodes. Thus I should either provide some new encode_fh variant
which would deal with inodes directly without "parents". Which doesn't
look for me anyhow better than the new export_encode_inode_fh helper.

After all, if the use of encode_fh become a mandatory rule we can easily
extend fsnotify fdinfo output to support new scheme without breaking
user space, because output looks like

 | fhandle-type:        1 f_handle: 49b1060023552153

(ie if something is changed than these fields will be simply updated).

Or maybe I miss something?

	Cyrill
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