Re: [PATCH 0/6] Sort out overlay layers and fs arrays

Amir Goldstein
2019-11-18T07:57:33Z

On Mon, Nov 18, 2019 at 8:03 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Sun, Nov 17, 2019 at 5:43 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > Miklos,
> >
> > When I started generalizing the lower_layers/lower_fs arrays
> > I noticed a bug that was introduced in v4.17 with xino.
> >
> > In the case of lower layer on upper fs, we do not have a pseudo_dev
> > assigned to lower layer and we expose the real lower st_dev;st_ino.
> > This happens on non-samefs when xino is disabled (default).
> > This is a very real bug, not really a corner case and I have an
> > an xfstest [1] for it that I will post later.
> >
> > In the mean while, I also pushed a fix to unionmount-testsuite devel
> > branch [2] to demonstrate the issue.
> >
> > With upstream kernel, this test ends up with a copied up file
> > from middle layer, whose on same fs as upper and its exposed
> > st_dev;st_ino are invalid:
> >
> > ./run --ov=1 --verify hard-link
> > ...
> > /mnt/a/no_foo110: File unexpectedly on upper layer
> >
> > Patch 1 in the series is a small fix for stable that fixes the
> > v4.17 regression in favor of a different, less severe regression.
> > The new regression can be demonstrated with:
> >
> > ./run --ov=1 --verify --xino hard-link
> > ...
> > /mnt/a/no_foo110: inode number/layer changed on copy up
> > (got 39:24707, was 39:24700)
> >
> > Patches 2-4 generalize the lower_{layer/fs} arrays to layer/fs arrays
> > and get rid of some special casing of upper layer.
> >
> > Patches 5-6 use the cleanup to solve the corner case that you pointed
> > out with bas_uuid [3] and to fix the regression introduced by patch 1.
> >
> > After patch 6, both unionmount-testsuite configurations
> > above pass the test st_dev;st_ino verifications.
> >
> > I doubt if patches 2-6 are stable material, because not sure the
> > corner cases they fix are worth the trouble.
> >
> > The series depends on the bad_uuid patch v5 that I posted on Thursday.
> >
> > I was also considering setting xino=on by default if xino_auto
> > is enabled, because what have we got to loose?
> >
> > The inodes whose st_ino fit in lower bits (by far more common) will
> > use overlay st_dev and the inodes whose st_ino overflow the lower bits
> > will use pseudo_dev. Seems like a win-win situation, but I wanted to
> > get your feedback on this before sending out a patch.
> >
>
> Arrr.. yes, there is a catch.
> Overflowing lower bits has a price beyond just using pseudo_dev.
> It introduces the possibility of inode number conflicts on directories,
> because directories never use pseudo_dev.
>

But we could mitigate that problem if we reserve an fsid for volatile
directory inode numbers. get_next_ino is 32bit anyway.
I am going to take a swing at having xino=auto always enabling xino.

Thanks,
Amir.