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

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Date:	Tue, 21 Aug 2012 16:12:56 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Pavel Emelyanov <xemul@...allels.com>,
	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel\@vger.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

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
> 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 ?

>
>
> 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,..) ?


>> (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

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