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

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Message-ID: <20120815220622.GA28054@fieldses.org>
Date:	Wed, 15 Aug 2012 18:06:23 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.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 01:02:37AM +0400, Cyrill Gorcunov wrote:
> On Wed, Aug 15, 2012 at 04:45:46PM -0400, J. Bruce Fields wrote:
> > On Wed, Aug 15, 2012 at 01:21:20PM +0400, Cyrill Gorcunov wrote:
> > > To provide fsnotify object inodes being watched without
> > > binding to alphabetical path we need to encode them with
> > > exportfs help. This patch adds a helper which operates
> > > with plain inodes directly.
> > 
> > I don't get it--this seems like a really roundabout way to get inode and
> > generation number, if that's all you want.
> 
> We can re-open the targets via filehandle on restore, this was the idea.
> All this series aimed to achieve the way to restore objects after checkpoit,
> thus we need to provide additional information which would be enough.

For this to work it'll need to be something you can pass to
open_by_handle_at, won't it?

> > 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.)

--b.
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