Re: [PATCH/RFC 00/11] expose btrfs subvols in mount table correctly

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 29/07/2021 02:39, NeilBrown wrote:
> On Wed, 28 Jul 2021, g.btrfs@cobb.uk.net wrote:
>> On 28/07/2021 08:01, NeilBrown wrote:
>>> On Wed, 28 Jul 2021, Wang Yugui wrote:
>>>> Hi,
>>>>
>>>> This patchset works well in 5.14-rc3.
>>>
>>> Thanks for testing.
>>>
>>>>
>>>> 1, fixed dummy inode(255, BTRFS_FIRST_FREE_OBJECTID - 1 )  is changed to
>>>> dynamic dummy inode(18446744073709551358, or 18446744073709551359, ...)
>>>
>>> The BTRFS_FIRST_FREE_OBJECTID-1 was a just a hack, I never wanted it to
>>> be permanent.
>>> The new number is ULONG_MAX - subvol_id (where subvol_id starts at 257 I
>>> think).
>>> This is a bit less of a hack.  It is an easily available number that is
>>> fairly unique.
>>>
>>>>
>>>> 2, btrfs subvol mount info is shown in /proc/mounts, even if nfsd/nfs is
>>>> not used.
>>>> /dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test
>>>> /dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test/sub1
>>>> /dev/sdc                btrfs   94G  3.5M   93G   1% /mnt/test/sub2
>>>>
>>>> This is a visiual feature change for btrfs user.
>>>
>>> Hopefully it is an improvement.  But it is certainly a change that needs
>>> to be carefully considered.
>>
>> Would this change the behaviour of findmnt? I have several scripts that
>> depend on findmnt to select btrfs filesystems. Just to take a couple of
>> examples (using the example shown above): my scripts would depend on
>> 'findmnt --target /mnt/test/sub1 -o target' providing /mnt/test, not the
>> subvolume; and another script would depend on 'findmnt -t btrfs
>> --mountpoint /mnt/test/sub1' providing no output as the directory is not
>> an /etc/fstab mount point for a btrfs filesystem.
> 
> Yes, I think it does change the behaviour of findmnt.
> If the sub1 automount has not been triggered,
>   findmnt --target /mnt/test/sub1 -o target
> will report "/mnt/test".
> After it has been triggered, it will report "/mnt/test/sub1"
> 
> Similarly "findmnt -t btrfs --mountpoint /mnt/test/sub1" will report
> nothing if the automount hasn't been triggered, and will report full
> details of /mnt/test/sub1 if it has.
> 
>>
>> Maybe findmnt isn't affected? Or maybe the change is worth making
>> anyway? But it needs to be carefully considered if it breaks existing
>> user interfaces.
>>
> I hope the change is worth making anyway, but breaking findmnt would not
> be a popular move.

I agree. I use findmnt, but I also use NFS mounted btrfs disks so I am
keen to see this deployed. But people who don't maintain their own
scripts and need a third party to change them might disagree!

> This is unfortunate....  btrfs is "broken" and people/code have adjusted
> to that breakage so that "fixing" it will be traumatic.
> 
> The only way I can find to get findmnt to ignore the new entries in
> /proc/self/mountinfo is to trigger a parse error such as by replacing the 
> " - " with " -- "
> but that causes a parse error message to be generated, and will likely
> break other tools.
> (...  or I could check if current->comm is "findmnt", and suppress the
> extra entries, but that is even more horrible!!)
> 
> A possible option is to change findmnt to explicitly ignore the new
> "internal" mounts (unless some option is given) and then delay the
> kernel update until that can be rolled out.

That sounds good as a permanent fix for findmnt. Some sort of
'--include-subvols' option. Particularly if it were possible to default
it using an environment variable so a script can be written to work with
both the old and the new versions of findmnt.

Unfortunately it won't help any other program which does similar
searches through /proc/self/mountinfo.

How about creating two different files? Say, /proc/self/mountinfo and
/proc/self/mountinfo.internal (better filenames may be available!). The
.internal file could be just the additional internal mounts, or it could
be the complete list. Or something like
/proc/self/mountinfo.without-subvols and
/proc/self/mountinfo.with-subvols and a sysctl setting to choose which
is made visible as /proc/self/mountinfo.

Graham