Regression in btrfs-next: BUG at fs/btrfs/super.c:984!

Regression in btrfs-next: BUG at fs/btrfs/super.c:984!

[Maciej Marcin Piechotka]

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After merging btrfs-next patches (chris' btrfs-3.0 branch) I get
following error:

[ 9799.199495] ------------[ cut here ]------------
[ 9799.199511] kernel BUG at fs/btrfs/super.c:984!
[ 9799.199524] invalid opcode: 0000 [#1] PREEMPT SMP 
[ 9799.199539] CPU 1 
[ 9799.199542] Modules linked in: cpufreq_stats btrfs zlib_deflate
libcrc32c microcode reiserfs arc4 snd_hda_codec_conexant uvcvideo
cdc_ether usbnet videodev v4l2_compat_ioctl32 cdc_acm cdc_wdm btusb
snd_hda_intel iwlagn bluetooth mac80211 snd_hda_codec snd_hwdep snd_pcm
cfg80211 snd_timer snd soundcore snd_page_alloc e1000e thinkpad_acpi
hwmon rfkill nvram zcache(PC) zram(C) rtc_cmos tp_smapi thinkpad_ec
psmouse fscache kvm_intel kvm loop configs ipv6 autofs4 uhci_hcd
firewire_ohci sdhci_pci firewire_core sr_mod ehci_hcd sdhci mmc_core
crc_itu_t cdrom sd_mod usbcore
[ 9799.199657] 
[ 9799.199669] Pid: 2263, comm: df Tainted: P         C  3.0.4-ck1+ #1
LENOVO 2242CTO/2242CTO
[ 9799.199686] RIP: 0010:[<ffffffffa032b682>]  [<ffffffffa032b682>]
btrfs_statfs+0x108/0x3a9 [btrfs]
[ 9799.199716] RSP: 0018:ffff8800414f9de8  EFLAGS: 00010246
[ 9799.199728] RAX: ffff880129bf8000 RBX: ffff8800414f9f00 RCX:
ffff880129bfa30c
[ 9799.199742] RDX: ffff880129c8e240 RSI: 000000009123683e RDI:
0000000000000000
[ 9799.199756] RBP: ffff880129bf8000 R08: ffffffff810d1ad0 R09:
0000000000000203
[ 9799.199769] R10: ffff88012254eab0 R11: 0000000000000203 R12:
ffff88011ba433c0
[ 9799.199783] R13: 0000000000000000 R14: 0000000000885fa0 R15:
ffff880129bfa368
[ 9799.199797] FS:  00007f15983f7700(0000) GS:ffff88013bc80000(0000)
knlGS:0000000000000000
[ 9799.199811] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 9799.199824] CR2: 00000000015fa008 CR3: 000000008e5d5000 CR4:
00000000000406e0
[ 9799.199838] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 9799.199851] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[ 9799.199865] Process df (pid: 2263, threadinfo ffff8800414f8000, task
ffff8800b8426780)
[ 9799.199878] Stack:
[ 9799.199889]  ffff8800414f9ed8 ffffffff810d1ad0 ffff88013162e4f8
ffffffff00000000
[ 9799.199906]  ffff880129f3c900 ffff88012cdf3800 ffff880129bf8000
0000000c430fe005
[ 9799.199923]  0000000000000000 ffff880132b0f540 ffff88011badc958
ffff880129bfa3c0
[ 9799.199940] Call Trace:
[ 9799.199955]  [<ffffffff810d1ad0>] ? user_path_at+0x55/0x7b
[ 9799.199971]  [<ffffffff810e5f2c>] ? statfs_by_dentry+0x46/0x5d
[ 9799.199985]  [<ffffffff810e5f56>] ? vfs_statfs+0x13/0x8b
[ 9799.199999]  [<ffffffff810d1ad0>] ? user_path_at+0x55/0x7b
[ 9799.200006]  [<ffffffff810e5ffe>] ? user_statfs+0x30/0x48
[ 9799.200006]  [<ffffffff810e6068>] ? sys_statfs+0x12/0x2b
[ 9799.200006]  [<ffffffff8136607b>] ? system_call_fastpath+0x16/0x1b
[ 9799.200006] Code: 18 48 89 33 4c 89 6b 20 48 89 43 08 48 8b 44 24 28
48 8b 80 20 01 00 00 48 8b 90 b8 23 00 00 48 89 44 24 30 8b 7a 30 85 ff
75 02 <0f> 0b 48 63 ff be 50 00 00 00 48 89 54 24 20 48 c1 e7 05 e8 56 
[ 9799.200006] RIP  [<ffffffffa032b682>] btrfs_statfs+0x108/0x3a9
[btrfs]
[ 9799.200006]  RSP <ffff8800414f9de8>
[ 9799.221655] ---[ end trace 327e64ade405e154 ]---

PS. It might happened that it was caused by parition mounted read-only
on read-only block device.

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Re: Regression in btrfs-next: BUG at fs/btrfs/super.c:984!

[David Sterba]

On Sat, Oct 01, 2011 at 11:43:10PM +0200, Maciej Marcin Piechotka wrote:
> PS. It might happened that it was caused by parition mounted read-only
> on read-only block device.
  ^^^

This is the key information!

super.c: btrfs_calc_avail_data_space()
 983         nr_devices = fs_info->fs_devices->rw_devices;
 984         BUG_ON(!nr_devices);


If there are no writable devices, this BUGON fires, although this case
in connection with RO mount is handled elsewhere properly.

This is called through sysfs and just tries to obtain available free
space information to print it. I think the BUG_ON is not necessary here
and the function can take a short track returning 0 as available free
space:

if (!nr_devices) {
	*free_bytes = 0;
	return 0;
}

I could be possible to actually read the free space even from a RO
device (be it RO mount or not), but I don't see much benefit which would
justify the work.

Can you please test the fix and let us know if the statfs number look
reasonable?


david

Re: Regression in btrfs-next: BUG at fs/btrfs/super.c:984!

[Maciej Marcin Piechotka]

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On Sun, 2011-10-02 at 19:39 +0200, David Sterba wrote:
> On Sat, Oct 01, 2011 at 11:43:10PM +0200, Maciej Marcin Piechotka wrote:
> > PS. It might happened that it was caused by parition mounted read-only
> > on read-only block device.
>   ^^^
> 
> This is the key information!
> 
> super.c: btrfs_calc_avail_data_space()
>  983         nr_devices = fs_info->fs_devices->rw_devices;
>  984         BUG_ON(!nr_devices);
> 
> 
> If there are no writable devices, this BUGON fires, although this case
> in connection with RO mount is handled elsewhere properly.
> 
> This is called through sysfs and just tries to obtain available free
> space information to print it. I think the BUG_ON is not necessary here
> and the function can take a short track returning 0 as available free
> space:
> 
> if (!nr_devices) {
> 	*free_bytes = 0;
> 	return 0;
> }
> 
> I could be possible to actually read the free space even from a RO
> device (be it RO mount or not), but I don't see much benefit which would
> justify the work.
> 
> Can you please test the fix and let us know if the statfs number look
> reasonable?
> 
> 
> david

Well - it shows size 50G, used 29G and avail 0G in df -h.

Regards

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