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

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Date:	Wed, 22 Aug 2012 11:19:07 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	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 08/21/2012 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.

open_by_handle use exportfs_decode_fh which use file system specific
fh_to_dentry

foe ext4 we require a generation number

		inode = get_inode(sb, fid->i32.ino, fid->i32.gen);

For brtfs

	objectid = fid->objectid;
	root_objectid = fid->root_objectid;
	generation = fid->gen;

	return btrfs_get_dentry(sb, objectid, root_objectid, generation, 1);

-aneesh

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