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

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Date:	Tue, 21 Aug 2012 14:49:47 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	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 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
>> 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.

>>
>> 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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