linux-kernel - Re: [overlayfs/port] overlayfs: v13 port attempt to kernel 3.5

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Date:	Mon, 20 Aug 2012 19:38:18 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	sedat.dilek@...il.com
Cc:	Andrew Watts <akwatts@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Daniel Baumann <daniel@...ian.org>
Subject: Re: [overlayfs/port] overlayfs: v13 port attempt to kernel 3.5


Sedat Dilek:
> The other part to run a Linux live-system is a "Union FileSystem" -
> this part is missing (speaking of upstream).
>
> Since years AUFS seems to be the choice #1 in a lot of distros to
> workaround the problem.
> NOTE: AUFS was rejected from upstream (to say not accepted).

Exactly.
The reason was that linux rejects all union-type filesystems but
UnionMount (which is union-type mount).
Later, the development of UnionMount seems to be almost stopped. The
essential design of overlayfs is based upon UnionMount, and I have
pointed out several issues such as
- for users, the inode number may change silently. eg. copy-up.
- hardlinks may break by copy-up.
- read(2) may get an obsoleted filedata (fstat(2) for metadata too).
- fcntl(F_SETLK) may be broken by copy-up.
- unnecessary copy-up may happen, for example mmap(MAP_PRIVATE) after
  open(O_RDWR).
- Later I noticed one more thing. /proc/PID/{fd/,exe} may not work
  correctly for overlayfs ...
- etc...
They are called "unPOSIXy behavior", and unforunately many of them are
NOT seem to be addressed in recent patches either.

Also I have posted
	If the development of UnionMount is really stopped, then I'd ask people
	to consider merging aufs as well as overlayfs.
but I am not sure whether LKML people are still waiting for UnionMount
and rejecting aufs.


J. R. Okajima
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