Re: [PATCH v2 00/20] Overlayfs inodes index

J. Bruce Fields
2017-06-07T18:59:01Z

On Wed, Jun 07, 2017 at 09:36:46PM +0300, Amir Goldstein wrote:
> On Wed, Jun 7, 2017 at 8:17 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> > On Wed, Jun 07, 2017 at 10:51:04AM +0300, Amir Goldstein wrote:
> >> Miklos,
> >>
> >> This set is an independent series a top of current overlayfs-next.
> >> I yanked all the dependencies from previous postings (consistent d_ino,
> >> dir_lock, verify_lower) and included everything needed in this posting.
> >>
> >> This work introcuding the inodes index opt-in feature, which provides:
> >> - Hardlinks are not broken on copy up
> >> - Infrastructure for overlayfs NFS export
> >
> > Minor suggestion: for the lazy readers among us, a quick summary of the
> > implementation might be helpful. (Could take most of it from just
> > 05/20?)
> >
>
> Heh, for the *very* lazy reader, I'll paste the referred snip from 05/20:
> "The index dir will contain hardlinks to upper inodes, named after the
> hex representation of their origin lower inodes.
> The index dir is going to be used to prevent breaking lower hardlinks
> on copy up and to implement overlayfs NFS export."
>
> That's me being very very brief about the what and the why, but nothing
> about the how, so here goes:
>
> The challenge with exporting overlayfs handled is that an object can start
> its life as a lower inode and transition into an upper inode (copy up).
> When the object is lower, the only way to encode it is by encoding the
> underlying fs lower inode, say encode(/lower/foo)=0x0001.
> The gist of the inodes index concept is that when decoding the handle,
> we need to check if the object was copied up.
> When an object is copied up, it creates an index entry at:
> workdir/index/0001, which is a hardlink to /upper/foo,
> so we have a way to get from the lower handle to the upper inode if it
> exists.
>
> The same lower -> upper mapping also helps herding a lower hardlinks
> bundle with their upper hardlinks bundle and refer to all as a single inode
> object. Most of the gritty details are in patches {08,11,18}/20.
>
> With this series, which is aiming for v4.13, NFS export is not there yet,
> because lower directories are not yet indexed (and more).
> I do, however, have a POC of non-persistent overlay NFS export [1]
> with a detailed documentation patch about the missing bits for persistent
> handles. I tested this POC with xfstest generic/426.
>
> Hope this summary was quick enough ;-)

Yes, thanks!

>
> Cheers,
> Amir.
>
> [1] https://github.com/amir73il/linux/commits/ovl-nfs-export-wip