archive mirror
 help / color / mirror / Atom feed
From: Amir Goldstein <>
To: Zhihao Cheng <>
Cc: Miklos Szeredi <>,
	overlayfs <>,
	Vivek Goyal <>
Subject: Re: [QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy?
Date: Sat, 7 Aug 2021 19:37:00 +0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, Aug 7, 2021 at 2:05 PM Amir Goldstein <> wrote:
> On Sat, Aug 7, 2021 at 1:17 PM Zhihao Cheng <> wrote:
> >
> > Hi, all.
> >
> > As title said. I wonder to know the reason for overlayfs mount failure
> > with '-o nfs_export=on,metacopy=on'.
> >
> > I modified kernel to enable these two options 'on',  it looks like that
> > overlayfs can still work fine under nfs_v4.
> >
> > Besides, I can get no more information about the reason from source
> > code, maybe I missed something.
> >
> It's because ovl_obtain_alias() (decoding a disconnected non-dir file handle)
> does not know how to construct a metacopy overlayfs inode.
> Maybe Vivek will be able to point you to the discussion that lead to making
> the features mutually exclusive.
> I don't remember any other reason.

I remembered some more details...

I think the main complication discussed w.r.t decoding a metacopy
inode was for the case where ovl_inode_lowerdata() differs from

If we had a weaker variant of metacopy (e.g. metacopy=upper) that
only allows creating and following metacopy inodes in the upper layer,
it would have been simpler to implement ovl_obtain_alias().

Specifically, when ofs->numlayer == 2 (single lower layer), there can
be no valid metacopy inodes in the lower layer, so that configuration
should also be rather easy to support.

Basically, for ovl_obtain_alias():
- 'lowerpath' must not have metadata xattr
- 'upper_alias' must not have metadata xattr
- If 'index' has metacopy xattr, OVL_UPPERDATA flag
  should not be set on ovl inode

But there are bigger complications w.r.t disconnected dentry.
Overlayfs knows how to decode, work with and copy up
disconnected dentries (parent is unknown), but in ovl_link(old, ...),
'old' dentry must not be a disconnected metacopy dentry when
calling ovl_set_link_redirect() => ... ovl_get_redirect(), so we will
also need to:
- On copy up of a disconnected lower, do not use metacopy
- Copy up data before ovl_link() when nfs_export is enabled
- In ovl_obtain_alias(), if 'index' has redirect:
-- Verify that it is an absolute path that it is resolved to the
'lowerpath's inode
-- oip.redirect needs to be passed to ovl_get_inode()
-- ovl_verify_inode() needs to verify that oip.redirect matches
   redirect that is found on existing ovl inode

And probably other things that I am forgetting...


  reply	other threads:[~2021-08-07 16:37 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-07 10:17 [QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy? Zhihao Cheng
2021-08-07 11:05 ` Amir Goldstein
2021-08-07 16:37   ` Amir Goldstein [this message]
2021-08-09  9:21     ` Zhihao Cheng
2021-08-09 21:35     ` Vivek Goyal
2021-08-10  4:17       ` Amir Goldstein
2021-08-10 19:48         ` Vivek Goyal
2021-08-10 20:00           ` Vivek Goyal
2021-08-11  6:51           ` Amir Goldstein
2021-08-16 18:27             ` Vivek Goyal
2021-08-16 18:44               ` Amir Goldstein
2021-08-16 19:00                 ` Vivek Goyal
2021-08-09 19:56   ` Vivek Goyal

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='' \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).