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

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Message-ID: <20120821120908.GB9483@fieldses.org>
Date:	Tue, 21 Aug 2012 08:09:09 -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>,
	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>,
	Al Viro <viro@...iv.linux.org.uk>,
	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 02:49:47PM +0400, Pavel Emelyanov wrote:
> On 08/21/2012 02:42 PM, Aneesh Kumar K.V wrote:
> > Pavel Emelyanov <xemul@...allels.com> writes:
> > 
> >> On 08/20/2012 11:32 PM, J. Bruce Fields wrote:
> >>> On Mon, Aug 20, 2012 at 11:06:06PM +0400, Cyrill Gorcunov wrote:
> >>>> On Mon, Aug 20, 2012 at 02:32:25PM -0400, J. Bruce Fields wrote:
> >>>>> On Mon, Aug 20, 2012 at 08:33:38PM +0400, Cyrill Gorcunov wrote:
> >>>>>> On Mon, Aug 20, 2012 at 07:49:23PM +0530, Aneesh Kumar K.V wrote:
> >>>>>>> Cyrill Gorcunov <gorcunov@...nvz.org> writes:
> >>>>>>>
> >>>>>>>> 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.
> >>>>>>>
> >>>>>>> doesn't name_to_handle_at()  work for you ? It also allows to get a file
> >>>>>>> handle using file descriptor.
> >>>>>>
> >>>>>> Hi, sorry for dealy. Well, the last idea is to get rid of this helper,
> >>>>>> I've sent out an updated version where ino+dev is only printed.
> >>>>>
> >>>>> I don't understand how ino and dev are useful to you, though, if you're
> >>>>> still hoping to be able to look up inodes using this information later
> >>>>> on.
> >>>>
> >>>> Hi Bruce, I believe having ino+dev is better than nothing. Otherwise we
> >>>> simply have no clue which targets are bound to inotify mark. Sometime
> >>>> (!) we can try to generate fhandle in userspace from this ino+dev bundle
> >>>> and then open the target file.
> >>>
> >>> That's insufficient to generate a filehandle in general.
> >>
> >> Yes, sure, but for live migration having inode and device is enough and that's why.
> >> We can use two ways of having a filesystem on the target machine in the same
> >> state (from paths points of view) as it was on destination one:
> >>
> >> 1. copy file tree in a rsync manner
> >> 2. copy a virtual disk image file
> >>
> >> In the 1st case we can map inode number to path easily, since we iterate over a filesystem

OK.  Then you don't care about unlinked files?

If the filesystem's frozen by the time you get here, I suppose you could
also just use paths?

> >> anyway. I agree, that rsync is not perfect for migration but still.
> >>
> >> In the 2nd case we can generate filehandle out of an inode number only since we _do_ know
> >> that inode will not get reused.
> > 
> > If you are going to to use open_by_handle, then that handle is not
> > sufficient right ? Or do you have open_by_inode ? as part of c/r ?
> 
> Why? For e.g. ext4 you can construct a handle in userspace and open by it.

If it's a real filehandle you want, in general you don't want to
construct it in userspace--depending on the filesystem it may require
filesystem-specific knowledge.

--b.

> 
> >>
> >> 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.
> 
> > 
> >>> (Also: there's the usual inode-number aliasing problem: the inode number
> >>> could get reused by another file.  Unless you know the file is being
> >>> held open the whole time.)
> >>>
> > 
> > -aneesh
> > 
> > .
> > 
> 
> Thanks,
> Pavel
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