Re: [QUESTION] Why overlayfs cannot mounted as nfs_export and metacopy?

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2021/8/8 0:37, Amir Goldstein 写道:
> 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
> ovl_inode_lower().
>
> 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, ...),

Can we get a disconnected non-dir dentry in normal process, or how to 
produce a disconnected dentry?

If I understand correctly, metacopy only inode will be processed in 
ovl_lower_fh_to_d():

706         /* First lookup overlay inode in inode cache by origin fh */
707         err = ovl_check_origin_fh(ofs, fh, false, NULL, &stack);
708         if (err)
709                 return ERR_PTR(err);
710
711         if (!d_is_dir(origin.dentry) ||
712             !(origin.dentry->d_flags & DCACHE_DISCONNECTED)) {
713                 inode = ovl_lookup_inode(sb, origin.dentry, false);
714                 err = PTR_ERR(inode);
715                 if (IS_ERR(inode))
716                         goto out_err;
717                 if (inode) {
718                         dentry = 
d_find_any_alias(inode);                        // A NULL dentry found 
here? How did it happen?
719 iput(inode);
720                         if (dentry)
721                                 goto out;
722 }
723         }

[...]

762         /* Get a connected non-upper dir or disconnected non-dir */
763         dentry = ovl_get_dentry(sb, NULL, &origin, index);      // 
Get disconnected non-dir.

> '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...
>
> Thanks,
> Amir.
> .