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

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Date:	Tue, 21 Aug 2012 08:29:08 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Tue, Aug 21, 2012 at 04:22:31PM +0400, Pavel Emelyanov wrote:
> On 08/21/2012 04:11 PM, J. Bruce Fields wrote:
> > On Tue, Aug 21, 2012 at 03:09:05PM +0400, Pavel Emelyanov wrote:
> >> On 08/21/2012 02:54 PM, Cyrill Gorcunov wrote:
> >>> On Tue, Aug 21, 2012 at 02:49:47PM +0400, Pavel Emelyanov wrote:
> >>>>>>
> >>>>>> However, if you have some better ideas on what information about inode should be exported
> >>>>>> to the userspace please share.
> >>>>>>
> >>>>>
> >>>>> Why not use name_to_handle(fd,...) and open_by_handle(handle,..) ?
> >>>>
> >>>> Because we don't have an fd at hands by the time we need to know the handle.
> >>>
> >>> Yeah, this might be not clear from patchset itself but inotify marks carry
> >>> inodes inside kernel thus it's inodes what we can use when we fetch information
> >>> about targets and put it into fdinfo output.
> >>
> >> Al, Bruce, Aneesh,
> >>
> >> What if we calculate the handle at the time we do have struct path at hands (i.e.
> >> when we create the inotify) and store it on the inotify structure purely to be 
> >> shown later in proc. Would that be acceptable?
> > 
> > Was it the lack of a dentry that was really the problem?  I thought it
> > was just the fact that not all filesystems support filehandles.
> 
> Initial problem -- we don't know what is being watched by an inotify fd.
> 
> Having a dentry somewhere was the 1st attempt to solve this -- keep a path
> in inotify and show it when required. It doesn't work since holding a ref on
> path changes the behavior of watched inode (we cannot rename/unlink/remount
> it the same way as we could before patching the kernel).

OK.  So if you don't mind the fact that there are filesystems with
inotify support but not filehandle support, then I think generating a
filehandle early as you describe would work.  I guess it's a little more
memory per watched inode.

--b.
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