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    SubjectRe: [autofs] [RFC] Towards a Modern Autofs
    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

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    NOTICE: The opinions expressed in this email are held by me,
    and may not represent the views of Sun Microsystems, Inc.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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