On Sat, Aug 7, 2021 at 2:05 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Sat, Aug 7, 2021 at 1:17 PM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> 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 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, ...), '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.
