Re: [Ksummit-discuss] [TECH TOPIC] Overlays and file(system) unioning issues

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2015-07-24 at 17:01 +0100, David Howells wrote:
> 
>  (1) Whiteouts.
> 
>      Linus's idea that a union layer or overlay mounted not as part of a union
>      but separately, should expose whiteouts as 0,0 chardevs.  Whilst this
>      might indeed make the backup tools easier as things like tar can then use
>      the stat() and mknod() interfaces rather than having to use special
>      ioctls or syscalls, Miklós's idea to implement them as actual 0,0
>      chardevs in the underlying filesystem incurs some problems:

Yeah, this is screwed up. I can kind of understand exposing them to
userspace as 0,0 chardevs in the case where they're mounted outside the
context of a union mount.

But these are *directory entries*, not real inodes. Using chardevs to
implement them *internally* is problematic. It's inefficient, because
it involves an inode lookup and uses up inode#s (in implementations
where that matters), and it's also caused deadlocks in some cases like
JFFS2 where we don't expect the recursion that it requires.

This was much nicer when it was being done with DT_WHT internally. And
you could *still* expose that to userspace as a 0,0 chardev — or
however you like.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation

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