linux-kernel - Re: [RFC PATCH] overlayfs: support more than one read-only layer

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Message-ID: <CAJfpegvYBMbhbmpA2kB01qQ3u5ict7rgWib4tx=941yyzqJBMQ@mail.gmail.com>
Date:	Mon, 17 Nov 2014 11:32:07 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Jordi Pujol Palomer <jordipujolp@...il.com>
Cc:	linux-unionfs@...r.kernel.org,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>, "A. Wan" <jm@...wan.com>,
	Patrick Frisch <patrick@...schux.de>,
	Aaron Campbell <aaron@...or.net>
Subject: Re: [RFC PATCH] overlayfs: support more than one read-only layer

On Fri, Nov 14, 2014 at 9:55 AM, Jordi Pujol Palomer
<jordipujolp@...il.com> wrote:
> EL Mon, 10 Nov 2014 10:09:54 +0100
> Miklos Szeredi <miklos@...redi.hu> escrigué:
>
>> Maybe it wasn't clear, but the number of lower layers isn't limited by
>> FILESYSTEM_MAX_STACK_DEPTH,
> sorry, you have been clear, it's me that have not explained the purpose
> of that patch. This idea is also valid for the main overlayfs
> development.
> Consider that we have an encrypted partition and a live OS that
> uses overlayfs and works over it, when the OS is started already
> reaches the maximum stacking depth (2). Also, hardcoding parameters
> into the code may be annoying because a change implies creating and
> mantaining patches, otherwise when the parameter is configurable we can
> change it in a config value.

Before allowing a change to FILESYSTEM_MAX_STACK_DEPTH we'd need to
analyze the kernel stack usage of anything taking part in the
filesystem stack.  Not something that can be expected of users before
they set a kernel config variable.

'Course we can change overlayfs and/or ecryptfs to use a new context
for operations done on underlying layers.  Not sure how much work that
would be...

> Also, I include a patch to explain better my first statement,
> would say lowerdirs has extended the functionality of lowerdir,
> therefore we can discard the lowerdir case that only works with one
> lower layer and rename lowerdirs to lowerdir. It's more simple and all
> the functionality is preserved.

Yeah.  But we also need some escaping in 'lowerdir=' argument to allow
filenames with comma and colon.  Current patch doesn't yet have that,
but will work on it.

Thanks,
Miklos
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