Re: [ANNOUNCE] bcachefs!

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On Fri, Jul 17, 2015 at 04:48:31PM -0700, Ming Lin wrote:
> 
> On Fri, 2015-07-17 at 16:40 -0700, Kent Overstreet wrote:
> > On Fri, Jul 17, 2015 at 04:35:55PM -0700, Ming Lin wrote:
> > > 
> > > On Fri, 2015-07-17 at 16:17 -0700, Kent Overstreet wrote:
> > > > On Wed, Jul 15, 2015 at 12:39:36AM -0700, Ming Lin wrote:
> > > > > On Wed, Jul 15, 2015 at 12:15 AM, Ming Lin <mlin@xxxxxxxxxx> wrote:
> > > > > > On Tue, 2015-07-14 at 23:58 -0700, Kent Overstreet wrote:
> > > > > >> Can you strace it?
> > > > > >
> > > > > > Strange. Now error message changed.
> > > > > 
> > > > > I mean sometimes it showed:
> > > > > 
> > > > > mount: /dev/sdt already mounted or /mnt/ busy
> > > > 
> > > > I have no idea what's going on, it works for me - is there anything unusual
> > > > about your setup? what kind of block device is /dev/sdt? is there any chance
> > > > there's another process that has it open? maybe try rebooting?
> > > 
> > > It's a regular HDD. I tried rebooting several times.
> > > 
> > > Now I try in qemu-kvm. Only the first time it can be mounted.
> > > 
> > > On host: qemu-img create hdd1.img 20G
> > > On guest: it's /dev/vda
> > > 
> > > root@block:~# bcacheadm format -C /dev/vda 
> > > UUID:			4730ed95-4c57-42db-856c-dbce36085625
> > > Set UUID:		e69ef0e0-0344-40d7-a6b1-c23d14745a32
> > > version:		6
> > > nbuckets:		40960
> > > block_size:		1
> > > bucket_size:		1024
> > > nr_in_set:		1
> > > nr_this_dev:		0
> > > first_bucket:		3
> > > 
> > > root@block:~# mount -t bcache /dev/vda /mnt/
> > > 
> > > root@block:~# mount |grep bcache
> > > /dev/vda on /mnt type bcache (rw,relatime)
> > > 
> > > root@block:~# reboot
> > > 
> > > root@block:~# dmesg |grep -i bcache
> > > [    2.548754] bcache: bch_journal_replay() journal replay done, 1 keys in 1 entries, seq 3
> > > [    2.636217] bcache: register_cache() registered cache device vda
> > > 
> > > 
> > > root@block:~# mount -t bcache /dev/vda /mnt/
> > > mount: No such file or directory
> > > 
> > > Now dmesg shows:
> > > 
> > > bcache: bch_open_as_blockdevs() register_cache_set err device already registered
> > 
> > Ohhhh.
> > 
> > The cache set is getting registered by the udev hooks. We should be able to
> > mount it anyways - same as you can mount any other fs in multiple locations.
> > 
> > I won't be able to fix this for at least a couple days, but for now - just
> > shut it down it via sysfs (echo 1 > /sys/fs/bcache/<uuid>/stop), then mount it.
> 
> It works!
> Any hint how to fix it? On udev or bcache-tool or kernel?
> I'd like to fix it.

The relevant code is in drivers/md/bcache/fs.c, bch_mount() ->
bch_open_as_blockdevs().

Part of the problem is that bcachefs isn't able to use much of the normal
generic mount path for block devices, partly because a fs can span multiple
block devices (same as btrfs).

I'm not sure the right way to fix it - it's going to take some thought, but
we want to do something like "is it already open? just take a ref on the
existing cache set".
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