* Re: bind mounts on btrfs cause mountinfo confusion
2017-07-17 21:47 bind mounts on btrfs cause mountinfo confusion Chris Murphy
@ 2017-07-18 7:24 ` Duncan
2017-07-18 18:57 ` Andrei Borzenkov
0 siblings, 1 reply; 3+ messages in thread
From: Duncan @ 2017-07-18 7:24 UTC (permalink / raw)
To: linux-btrfs
Chris Murphy posted on Mon, 17 Jul 2017 15:47:20 -0600 as excerpted:
> Example during boot:
>
> Executing: /usr/bin/mount
> /sysroot/ostree/deploy/fedora-workstation/var /var -o bind
>
> cat /proc/self/mounts
>
> /dev/sda6 /var btrfs
> rw,relatime,ssd,space_cache,subvolid=5,subvol=/ostree/deploy/fedora-
> workstation/var 0 0
>
> findmnt shows basically the same [options only listed]
>
> rw,relatime,ssd,space_cache,subvolid=5,subvol=/ostree/deploy/fedora-
> workstation
>
>
> There is no such subvolume /ostree/deploy/fedora-workstation/var,
> basically it seems like whatever is producing this mount information
> (kernel code?) is just assuming a bind mount on Btrfs is a subvolume
> without checking if that subvolume really exists, and if it's just an
> ordinary directory being bind mounted, which is what's going on here.
>
> I'd say it's confusing. But I don't know if it's a problem otherwise.
Yes, it's kernel code. The reason is that btrfs internally handles
subvolume mounts as bind-mounts, so it thinks all bind-mounts are
subvolume mounts, even when they're not.
It can be slightly confusing, true, but I actually appreciate having the
path information available here for my bind-mounts (which aren't
subvolumes as I don't use subvolumes here). It makes it easier to figure
out which are the bind-mounts vs the original mount. From my mount
output:
/dev/sda5 on / type btrfs (..,subvolid=5,subvol=/)
/dev/sda5 on /m/cbind/etc/bind type btrfs (..,subvolid=5,subvol=/etc/bind)
/dev/sda6 on /h type btrfs (..,subvolid=5,subvol=/)
/dev/sda6 on /m/cbind/var/bind type btrfs (..,subvolid=5,subvol=/var/bind)
Note the subvolid=5. For real subvolume mounts that should be something
else, so it makes it clear it's the root subvolume and thus a regular
bind-mount not a subvolume mount, regardless of what subvol= says.
Meanwhile, the subvol= gives me the information that the duplicate
/dev/sdx is missing, the path on the device that's actually being
mounted. For the original mount it's /. For bind-mounts, it's the path
that's being bind-mounted elsewhere, /etc/bind being bind-mounted to
/m/cbind/etc/bind, for the first (sda5) listing above.
Before the kernel started printing that subvol= stuff, this information
was missing, as it still is for non-btrfs.
So while slightly confusing, I definitely /like/ having that new
information available. =:^)
Arguably it could be printed as something other than "subvol=", however,
but what? If I had a reasonable suggestion for something that retains
the additional information but looks less confusing regardless of whether
it's an actual subvolume mount or just a bind-mount, I'd have made it
already. =:^(
--
Duncan - List replies preferred. No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master." Richard Stallman
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