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

[Chris Murphy <lists@colorremedies.com>]

On Sat, Nov 4, 2017 at 12:27 PM, Andrei Borzenkov <arvidjaar@gmail.com> wrote:
> 04.11.2017 10:05, Adam Borowski пишет:
>> 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
>>
>
> Indeed. I can build the same configuration on non-btrfs and updatedb
> would skip non-overlapping mounts just as it would on btrfs. It is just
> that it is rather more involved on other filesystems (and as you
> mentioned this requires top-level to be mounted at some point), while on
> btrfs it is much easier to get (and is default on number of distributions).
>
> So yes, it really appears that updatedb check for duplicated mounts is
> wrong in general and needs rethinking.

Yes, even if it's not a Btrfs bug, I think it's useful to get a
different set of eyes on this than just the mlocate folks. Maybe it
should get posted to fs-devel?

-- 
Chris Murphy