linux-kernel - Re: [PATCH 0/7] overlay filesystem: request for inclusion

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Message-ID: <18299.1308193639@jrobl>
Date:	Thu, 16 Jun 2011 12:07:19 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Erez Zadok <ezk@....cs.sunysb.edu>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Valerie Aurora <val@...consulting.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	NeilBrown <neilb@...e.de>,
	"viro@...IV.linux.org.uk" <viro@...IV.linux.org.uk>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"nbd@...nwrt.org" <nbd@...nwrt.org>,
	"hramrach@...trum.cz" <hramrach@...trum.cz>,
	"jordipujolp@...il.com" <jordipujolp@...il.com>
Subject: Re: [PATCH 0/7] overlay filesystem: request for inclusion


Erez Zadok:
> ...  Asking =
> overlayfs or other stackable file systems to solve this multi-layer =
> coherency perfectly is somewhat ridiculous: we don't expect file systems =
> like ext3 to detect and correctly handle changes to lower devices =97 =
> i.e., if someone hand-edited direct blocks in /dev/sda1, do we?

I agree with you if we discuss about union-type-mount, which handles a
block device as its member. As long as the layered-fs handles a
directory (mounted filesystem) as its member, it is obviously right that
users expect the modification on the member fs (by-passing a union) is
available.

Of course I agree it brings complication to us, and I'd suggest three
level options to support this issue.
- detect the direct changes and reflect it to union (hardest option)
- skip the detection, but verify the parent-child relationship or more
  at least. (this is something like overlayfs is trying to do)
- skip both of the detection and verification (lowest option)
  this option depends how user sets up the union and its member. if user
  hides the members totally by over-mounting an empty dir on the member
  (or something), then he can specify this option. otherwise, this
  option is dangerous. also some symlinks may not work.
  # mkdir /hide
  # mount -o upper=/rw,lower=/ro none /union
  # mount -o bind /hide /rw
  # mount -o bind /hide /ro


J. R. Okajima
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