Re: [RFC] Towards a Modern Autofs

[Mike Waychison <Michael.Waychison@Sun.COM>]

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H. Peter Anvin wrote:

>Mike Waychison wrote:
>  
>
>>>Special vfsmount mounted somewhere; has no superblock associated with it;
>>>attempt to step on it triggers event; normal result of that event is to
>>>get a normal mount on top of it, at which point usual chaining logics
>>>will make sure that we don't see the trap until it's uncovered by removal
>>>of covering filesystem.  Trap (and everything mounted on it, etc.) can
>>>be removed by normal lazy umount.
>>>
>>>Basically, it's a single-point analog of autofs done entirely in VFS.
>>>The job of automounter is to maintain the traps and react to events.
>>>
>>>      
>>>
>>Is there any clear advantage to doing this in the VFS other than saving
>>a superblock and a dentry/inode pair or two?
>>
>>I remember talking to you about this, and I seem to recall that these
>>mount traps would probably communicate using a struct file, so a
>>trap-user would somehow receive events about when the trap was set
>>off.   Will this communication model continue to work within a cloned
>>namespace?  What happens if the trap-client closes the file?
>>
>>    
>>
>
>The biggest issue is to ensure that the appropriate atomicity guarantees
>can be maintained.  In particular, it must be possible to umount the
>underlying filesystem and all mount traps on top of it atomically.
>Anything less will result in race conditions.
>
>	-hpa
>
>  
>
Unless I'm missing something, implementing this as a seperate filesystem 
type still has the appropriate atomicity guarantees as long as the VFS 
support complex expiry, whereby userspace would tag submounts as being 
part of the overall expiry for a base mountpoint.

-- 
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice
mailto: Michael.Waychison@Sun.COM
http://www.sun.com

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