Re: [PATCH v5 2/4] ovl: allocate anonymous devs for lowerdirs

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Nov 01, 2017 at 06:41:51PM +0200, Amir Goldstein wrote:

[..]
> >> I can't remember if there was a reason for not allocating anonymous bdev
> >> for upper
> >
> > That's a good point.
> >
> >> or if it just because we did not need it to guaranty uniqueness
> >> of st_dev/st_ino *among* overlay inodes
> >
> > Even for lower, st_dev will be unique for different lower on non same-fs,
> > right. IOW, when it come to uniqueness of st_dev/st_ino pair, among
> > overlay inodes, lower and upper should have same requirements.
> >
> >> while guarantying constant
> >> st_dev/st_ino across copy up.
> >
> > Hmm..., I did not get this point. Over copy up, atleast st_ino will change
> > for non-samefs case.
> >
> > I will spend more time on patch.
> >
> 
> Urgh! It took me a while to remember the reason why system wide uniqueness
> is important for lower but less for upper.
> An upper object has the same content as the "real" object and they have the
> same st_ino/st_dev so its ok that diff will skip comparing them.
> A copy-up object does not have the same content as the lower "real" object,
> so if it has the same st_ino/st_dev as real object, diff will skip compare and
> we have a problem.

I am not sure I understand this.  So you are doing a diff between a file
on overlayfs and same file accessed outside overlayfs?

If a file is on lower, then it has not been modified and diff skipping
it makes perfect sense?

Vivek