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

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Date:	Tue, 21 Aug 2012 13:18:25 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
CC:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"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 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
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.


However, if you have some better ideas on what information about inode should be exported
to the userspace please share.

> (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.)
> 
> --b.
> .
> 

Thanks,
Pavel
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