system hangs due to qgroups

[Marc Joliet <marcec@gmx.de>]

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Hello all,

I'm having some trouble with btrfs on a laptop, possibly due to qgroups.  
Specifically, some file system activities (e.g., snapshot creation, 
baloo_file_extractor from KDE Plasma) cause the system to hang for up to about 
40 minutes, maybe more.  It always causes (most of) my desktop to hang, 
(although I can usually navigate between pre-existing Konsole tabs) and 
prevents new programs from starting.  I've seen the system load go up to >30 
before the laptop suddenly resumes normal operation.  I've been seeing this 
since Linux 4.7, maybe already 4.6.

Now, I thought that maybe this was (indirectly) due to an overly full file 
system (~90% full), so I deleted some things I didn't need to get it up to 15% 
free.  (For the record, I also tried mounting with ssd_spread.)  After that, I 
ran a balance with -dusage=50, which started out promising, but then went back 
to the "bad" behaviour.  *But* it seemed better than before overall, so I 
started a balance with -musage=10, then -musage=50.  That turned out to be a 
mistake.  Since I had to transport the laptop, and couldn't wait for "balance 
cancel" to return (IIUC it only returns after the next block (group?) is 
freed), I forced the laptop off.

After I next turned on the laptop, the balance resumed, causing bootup to 
fail, after which I remembered about the skip_balance mount option, which I 
tried in a rescue shell from an initramfs.  But wait, that failed, too!  
Specifically, the stack trace I get whenever I try it includes as one of the 
last lines:

"RIP [<ffffffff8131226f>] qgroup_fix_relocated_data_extents+0x1f/0x2a8"

(I can take photos of the full stack trace if requested.)

So then I ran "btrfs qgroup show /sysroot/", which showed many quota groups, 
much to my surprise.  On the upside, at least now I discovered the likely 
reason for the performance problems.

(I actually think I know why I'm seeing qgroups: at one point I was trying out 
various snapshot/backup tools for btrfs, and one (I forgot which) 
unconditionally activated quota support, which infuriated me, but I promptly 
deactivated it, or so I thought.  Is quota support automatically enabled when 
qgroups are discovered, or did I perhaps not disable quota support properly?)

Since I couldn't use skip_balance, and logically can't destroy qgroups on a 
read-only file system, I decided to wait for a regular mount to finish.  That 
has been running since Tuesday, and I am slowly growing impatient.

Thus I arrive at my question(s): is there anything else I can try, short of 
reformatting and restoring from backup?  Can I use btrfs-check here, or any 
other tool?  Or...?

Also, should I be able to avoid reformatting: how do I properly disable quota 
support?

(BTW, searching for qgroup_fix_relocated_data_extents turned up the ML thread 
"[PATCH] Btrfs: fix endless loop in balancing block groups", could that be 
related?)

The laptop is currently running Gentoo with Linux 4.8.10 and btrfs-progs 
4.8.4.

Greetings
-- 
Marc Joliet
--
"People who think they know everything really annoy those of us who know we
don't" - Bjarne Stroustrup

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