Re: [PATCH v2 0/6] Btrfs: show subvolume name and ID in /proc/mounts

[Omar Sandoval]

On Thu, Apr 09, 2015 at 02:34:50PM -0700, Omar Sandoval wrote:
> Here's version 2 of providing the subvolume name and ID in /proc/mounts.
> 
> It turns out that getting the name of a subvolume reliably is a bit
> trickier than it would seem because of how mounting subvolumes by ID is
> implemented. In particular, in that case, the dentry we get for the root
> of the mount is not necessarily attached to the dentry tree, which means
> that the obvious solution of just dumping the dentry does not work. The
> solution I put together makes the tradeoff of churning a bit more code
> in order to avoid implementing this with weird hacks.
> 
> Changes from v1 (https://lkml.org/lkml/2015/4/8/16):
> 
> - Put subvol= last in show_options
> - Change commit log to remove comment about userspace having no way to
>   know which subvolume is mounted, as David pointed out you can use
>   btrfs inspect-internal rootid <mountpoint>
> - Split up patch 2
> - Minor coding style fixes
> 
> This still applies to v4.0-rc7. Tested manually and with the script
> below (updated from v1).
> 
> Thanks!
> 
> Omar Sandoval (6):
>   Btrfs: lock superblock before remounting for rw subvol
>   Btrfs: remove all subvol options before mounting top-level
>   Btrfs: clean up error handling in mount_subvol()
>   Btrfs: fail on mismatched subvol and subvolid mount options
>   Btrfs: unify subvol= and subvolid= mounting
>   Btrfs: show subvol= and subvolid= in /proc/mounts
> 
>  fs/btrfs/super.c | 376 ++++++++++++++++++++++++++++++++++++-------------------
>  fs/seq_file.c    |   1 +
>  2 files changed, 251 insertions(+), 126 deletions(-)
> 

Hi, everyone,

Just wanted to revive this so we can hopefully come up with a solution
we agree on in time for 4.2.

Just to recap, my approach (and also Qu Wenruo's original approach) is
to convert subvolid= mounts to subvol= mounts at mount time, which makes
showing the subvolume in /proc/mounts easy. The benefit of this approach
is that looking at mount information, which is supposed to be a
lightweight operation, is simple and always works. Additionally, we'll
have the info in a convenient format in /proc/mounts in addition to
/proc/$PID/mountinfo. The only caveat is that a mount by subvolid can
fail if the mount races with a rename of the subvolume.

Qu Wenruo's second approach was to instead convert the subvolid to a
subvolume path when reading /proc/$PID/mountinfo. The benefit of this
approach is that mounts by subvolid will always succeed in the face of
concurrent renames. However, instead, getting the subvolume path in
mountinfo can now fail, and it makes what should probably be a
lightweight operation somewhat complex.

In terms of the code, I think the original approach is cleaner: the
heavy lifting is done when mounting instead of when reading a proc file.
Additionally, I don't think that the concurrent rename race will be much
of a problem in practice. I can't imagine that too many people are
actively renaming subvolumes at the same time as they are mounting them,
and even if they are, I don't think it's so surprising that it would
fail. On the other hand, reading mount info while renaming subvolumes
might be marginally more common, and personally, if that failed, I'd be
unpleasantly surprised.

Orthogonal to that decision is the precedence of subvolid= and subvol=.
Although it's true that mount options usually have last-one-wins
behavior, I think David's argument regarding the principle of least
surprise is solid. Namely, someone's going to be unhappy with a
seemingly arbitrary decision when they don't match.

Sorry for the long-winded email! Thoughts, David, Qu?

Thanks,
-- 
Omar
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