Re: partially encrypted filesystem

From: Linus Torvalds <torvalds@osdl.org>
To: Jeff Garzik <jgarzik@pobox.com>
Cc: "Jörn Engel" <joern@wohnheim.fh-wedel.de>,
	linux-kernel@vger.kernel.org,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: partially encrypted filesystem
Date: Wed, 3 Dec 2003 18:08:53 -0800 (PST)	[thread overview]
Message-ID: <Pine.LNX.4.58.0312031807080.2055@home.osdl.org> (raw)
In-Reply-To: <3FCE8CF5.4030006@pobox.com>

On Wed, 3 Dec 2003, Jeff Garzik wrote:
> Linus Torvalds wrote:
> >
> > And that in turn causes problems. You get all kinds of interesting
> > deadlock schenarios when write-out requires more memory in order to
> > succeed. So you need to get careful. Reading ends up being the much easier
> > case (doesn't have the same deadlock issues _and_ you could do it in-place
> > anyway).
>
>
> FWIW zisofs and ntfs have to do this too, since X on-disk compressed
> pages must be expanded to X+Y in-memory pages...

Note how zisofs only has to handle the read-only case. cramfs does the
same.

Reading is trivially done by having a few extra buffers (either through
the buffer cache or explicit buffering).

Writing is the more interesting case. I don't know how well NTFS does in
low-memory circumstances.

(Which is by no means to say that it is somehow impossible to get right:
you just have to be careful).

		Linus