Re: updatedb does not index /home when /home is Btrfs

[Adam Borowski <kilobyte@angband.pl>]

On Sat, Nov 04, 2017 at 09:26:36AM +0300, Andrei Borzenkov wrote:
> 04.11.2017 07:49, Adam Borowski пишет:
> > On Fri, Nov 03, 2017 at 06:15:53PM -0600, Chris Murphy wrote:
> >> Ancient bug, still seems to be a bug.
> >> https://bugzilla.redhat.com/show_bug.cgi?id=906591
> >>
> >> The issue is that updatedb by default will not index bind mounts, but
> >> by default on Fedora and probably other distros, put /home on a
> >> subvolume and then mount that subvolume which is in effect a bind
> >> mount.
> >>
> >> There's a lot of early discussion in 2013 about it, but then it's
> >> dropped off the radar as nobody has any ideas how to fix this in
> >> mlocate.
> > 
> > I don't see how this would be a bug in btrfs.  The same happens if you
> > bind-mount /home (or individual homes), which is a valid and non-rare setup.
> 
> It is the problem *on* btrfs because - as opposed to normal bind mount -
> those mount points do *not* refer to the same content.

Neither do they refer to in a "normal" bind mount.

> As was commented in mentioned bug report:
> 
> mount -o subvol=root /dev/sdb1 /root
> mount -o subvol=foo /dev/sdb1 /root/foo
> mount -o subvol bar /dev/sdb1 /bar/bar
> 
> Both /root/foo and /root/bar, will be skipped even though they are not
> accessible via any other path (on mounted filesystem)

losetup -D
truncate -s 4G junk
losetup -f junk
mkfs.ext4 /dev/loop0
mkdir -p foo bar
mount /dev/loop0 foo
mkdir foo/bar
touch foo/fileA foo/bar/fileB
mount --bind foo/bar bar
umount foo

> It is a problem *of* btrfs because it does not offer any easy way to
> distinguish between subvolume mount and bind mount. If you are aware of
> one, please comment on mentioned bug report.

Well, subvolume mounts are indistinguishable from bind mounts because they
_are_ bind mounts.  You merely don't need to mount the "master" first.

The only way such a "master" mount is special is that, on most filesystems,
its root was accessible at least at some point (but it might no longer be,
thanks to chroot, pivot_root, etc).


Meow!
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