Re: [RFC][PATCH 1/8] namespace: use global namespace semaphore

[Mike Waychison <mike@waychison.com>]

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Miklos Szeredi wrote:
>>  - this triggers the actual mount in the daemon, which mounts over
>>    the autofs filesystem (with some trick to avoid recursion)
> 
> 
> BTW, there's an elegant way to avoid recursion:
> 
>  - _before_ mounting the autofs, do mpfd = open(mountpoint, O_RDONLY).
> 
>  - then mount autofs
> 
>  - when mount trigger comes in do:  fchdir(mpfd), mount whatever .
> 
> This way the autofs mount won't be followed by the mount program,
> since looking up '.' does not follow mounts.
> 
> So the result is two sibling mounts: the original autofs and the
> actual filesystem.  But subsequent lookups will see the later mount
> only, since it's placed earlier on the hash chain, and lookup_mnt() will
> always return that.

Indeed.  This is how autofsng handles direct mounts and 'browse' mounts
currently, although it currently uses mountfds to do a move in the end
(avoiding having to hijack cwd to do the final move).  This could be
better achieved by mount --move'ing to /proc/self/fd/#.

Mike Waychison
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