Re: Regression in handling power cuts since 3a1e819b4e80 ("ovl: store file handle of lower inode on copy up")

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Oct 19, 2018 at 5:59 PM Richard Weinberger <richard@nod.at> wrote:
>
> Am Freitag, 19. Oktober 2018, 16:45:53 CEST schrieb Richard Weinberger:
> > Well, I fear it uncovers a problem in UBIFS. We had already problems with overlayfs.
> > Did you bisect the problem and you are sure that the said commit is the first bad commit?
>
> Another thing, UBIFS has no export operations, so overlayfs will fall back to xattrs,
> if I read the commit correctly.
> Maybe this is the root of the problem.
>

You are not reading the commit correctly.

Commit says:
    "If lower fs does not support NFS export ops store a zero sized xattr so we
    can always use the overlay.origin xattr to distinguish between a copy up
    and a pure upper inode."

What it means is:
- Doesn't matter if ubifs supports NFS export - it's not the lower fs
- squashfs supports NFS export, but has zero s_uuid, so currently that
  is treated the same as no NFS export support (I have patch to relax this)
- The result of no lower NFS export support is that a zero size xattr is written
  as opposed to non-zero size xattr in the case of lower support NFS export

Thanks,
Amir.