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

[Miklos Szeredi]

On Fri, Nov 14, 2014 at 9:55 AM, Jordi Pujol Palomer
<jordipujolp@xxxxxxxxx> wrote:
> EL Mon, 10 Nov 2014 10:09:54 +0100
> Miklos Szeredi <miklos@xxxxxxxxxx> escriguÃ:
>
>> 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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