Re: [patch 4/8] fs, exportfs: Add export_encode_inode_fh helper

From: "J. Bruce Fields" <bfields@fieldses.org>
To: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>,
	James Bottomley <jbottomley@parallels.com>,
	Pavel Emelianov <xemul@parallels.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Matthew Helsley <matt.helsley@gmail.com>
Subject: Re: [patch 4/8] fs, exportfs: Add export_encode_inode_fh helper
Date: Thu, 16 Aug 2012 10:54:34 -0400	[thread overview]
Message-ID: <20120816145434.GI28054@fieldses.org> (raw)
In-Reply-To: <20120816144835.GC13066@moon>

On Thu, Aug 16, 2012 at 06:48:35PM +0400, Cyrill Gorcunov wrote:
> On Thu, Aug 16, 2012 at 03:41:52PM +0100, Al Viro wrote:
> > On Thu, Aug 16, 2012 at 06:15:53PM +0400, Cyrill Gorcunov wrote:
> > > On Thu, Aug 16, 2012 at 02:03:00PM +0000, James Bottomley wrote:
> > > > > > What's wrong with saying "we don't support idiotify"?
> > > > > 
> > > > > Al, we need some way to restore inotifies after checkpoint.
> > > > > At the very early versions of these patches I simply added
> > > > > dentry to the inotify mark thus once inotify created we always
> > > > > have a dentry to refer on in encode_fh, but I'm not sure if
> > > > > this will be good design.
> > > > 
> > > > Actually, I was about to suggest this.  This can be done internally
> > > > within fs/notify without actually modifying the syscall interface, can't
> > > > it, since they take a path which is used to obtain the inode?  It looks
> > > > like the whole of the inotify interface could be internally recast to
> > > > use dentries instead of inodes.  Unless I've missed something obvious?
> > > 
> > > Well, after looking into do_sys_name_to_handle->exportfs_encode_fh
> > > sequence more precisely it seems it will be easier to extend
> > > exportfs_encode_fh to support inodes directly instead of playing
> > > with notify code (again, if i'm not missing something too).
> > > i'm cooking a patch to show (once it's tested i'll send it out).
> > 
> > Good luck doing that with e.g. VFAT...  And then there's such thing
> > as filesystems that don't have ->encode_fh() for a lot of very good
> 
> Wait, Al, it seems I messed up. If some fs has no encode_fh() implemented
> the default encoding with FILEID_INO32_GEN_PARENT will be used for that.

Yeah, but then try decoding it; the first two lines of exportfs_decode_fh:

	if (!nop || !nop->fh_to_dentry)
		return ERR_PTR(-ESTALE);

Fundamentally, if you're asking for something that you can use to look up an
inode on a filesystem (and that works even after the inode's diseappeared from
the inode cache), then you're asking for a filehandle.  Filesystems that
currently don't support filehandles probably lack that support for some good
reason.

--b.

> 
> > reasons; just try to do that on sysfs, for example.  Or on ramfs,
> > for that matter...  And while saying "you can't export that over
> > NFS" seems to work fine, idiotify-lovers will screech if you try
> > to ban their perversion of choice on those filesystems.
> 
> 	Cyrill