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

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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.
.





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