linux-kernel - DT_WHT (Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16))

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Date:	Thu, 14 Mar 2013 21:57:46 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Robo Bot <apw@...onical.com>, Felix Fietkau <nbd@...nwrt.org>,
	Neil Brown <neilb@...e.de>,
	Jordi Pujol <jordipujolp@...il.com>, ezk@....cs.sunysb.edu,
	David Howells <dhowells@...hat.com>,
	Sedat Dilek <sedat.dilek@...glemail.com>
Subject: DT_WHT (Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16))


Miklos Szeredi:
> On Thu, Mar 14, 2013 at 12:19 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
	:::
> > As for whiteouts... I think we ought to pull these bits of unionmoun
> > queue into the common stem and add the missing filesystems to them;
> > ext* and ufs are trivial (keep in mind that FFS derivatives, including
> > ext*, have d_type in directory entry and type 14 (DT_WHT) is there
> > precisely for that purpose).  btrfs also has "dir_type" thing - 8bit
> > field...
>
> What about userspace interfaces?  Are we allowed to extend d_type and
> st_mode without breaking things?

Introducing a new d_type value can be a headache for other filesystems
or other OSs. Whiteout and opaque by xattr is an idea, but I prefer more
primitive way (special hidden filename) since xattr may force users to
change their configuration and it consumes disk space a little.
Filesystems may have a limit for the consumed size by xattr. If xattr
reaches the limit, then several operation in overlayfs will be unusable.


J. R. Okajima
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