Re: [PATCH v2 4/5] ovl: Support creation of whiteout files on overlayfs

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi folks,

On 2023/8/23 01:29, Amir Goldstein wrote:
> On Tue, Aug 22, 2023 at 7:19 PM Miklos Szeredi <miklos@szeredi.hu> wrote:

...

>>
> 
> I proposed to look at the two features independently:
> 1. Nesting of overlayfs xattrs (patch 3/5)
> 2. Alternative format for whiteout (overlay.whiteout) that can be used
>     by container tools converting OCI/tar images to overlayfs images
> 
> Together, they provide a solution to Alexander's use case.
> 
> IIUC, the way this is intended to work is that mkfs.composefs
> is running inside a container, whose work directory is overlayfs
> and it composes some lower layers on that host mounted overlayfs.
> 
> mkfs.composefs composes layers with overlay.{metacopy,whiteout,redirect}
> xattrs (up to here it is standard mkfs.composefs) and because those layers
> are stored in overlayfs, the xattrs are stored in the host fs as
> overlay.overlay.*.
> 
> I hope I got the use case correctly?
Sorry for the dumb questions below.  I'm interested in the use cases:
after checking the previous github issue and emails (sorry if I'm
still missing something), I'm curious about this too.

I totally understand how it plans to work and how it works (by using
escape xattr prefixes) but I'm not sure if I'm quite understand the
original issue:

Do composefs use cases store overlayfs xattrs in the meta-only layer?
if so, such layer is actually hand-crafted by mkfs.  Why do we need
a way to keep escape xattrs on the underlay overlayfs?  Does the
other layer are data-only layers (do we keep some overlay xattrs in
these data-only layers)?

Thanks,
Gao Xiang