============================================================
[ 3756.152833] BUG: unable to handle kernel NULL pointer dereference at 00000000000001f0
[ 3756.152882] IP: [<ffffffffa01d9c21>] can_overcommit+0x21/0xf0 [btrfs]
[ 3756.152936] PGD 0
[ 3756.152949] Oops: 0000 [#1] SMP
[ 3756.152969] Modules linked in: ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_filter ebtable_broute bridge stp llc ebtable_nat 
ebtables ip6table_mangle ip6table_raw ip6table_security ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_filter ip6_tables 
iptable_mangle iptable_raw iptable_security iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack coretemp kvm_intel kvm crc32
_pclmul iTCO_wdt iTCO_vendor_support microcode ipmi_si lpc_ich mfd_core pcspkr acpi_power_meter ipmi_msghandler i2c_i801 i7core_edac shpchp edac_core 
nfsd acpi_cpufreq auth_rpcgss nfs_acl lockd grace sunrpc sch_fq_codel btrfs xor raid6_pq usb_storage mgag200 drm_kms_helper syscopyarea sysfillrect 
sysimgblt fb_sys_fops ttm drm igb ptp ata_generic pps_core pata_acpi crc32c_intel
[ 3756.153397]  dca megaraid_sas i2c_algo_bit ata_piix i2c_core
[ 3756.153433] CPU: 3 PID: 3004 Comm: kworker/u25:4 Tainted: G          I     4.3.0-rc6 #1
[ 3756.153469] Hardware name: FUJITSU-SV                       PRIMERGY RX300 S6             /D2619, BIOS 6.00 Rev. 1.09.2619.N1           12/13/2010
[ 3756.153537] Workqueue: events_unbound btrfs_async_reclaim_metadata_space [btrfs]
[ 3756.153571] task: ffff88023581a400 ti: ffff880234648000 task.ti: ffff880234648000
[ 3756.153604] RIP: 0010:[<ffffffffa01d9c21>]  [<ffffffffa01d9c21>] can_overcommit+0x21/0xf0 [btrfs]
[ 3756.153655] RSP: 0018:ffff88023464bda8  EFLAGS: 00010282
[ 3756.153679] RAX: 0000000001000000 RBX: ffff880431f68c00 RCX: 0000000000000002
[ 3756.153711] RDX: 0000000000c00000 RSI: 0000000000000000 RDI: 0000000000000000
[ 3756.153742] RBP: ffff88023464bde0 R08: 0000000000000101 R09: 000000000000000c
[ 3756.153773] R10: ffffffff81d10060 R11: ffffffff81d10050 R12: ffff880431f68c00
[ 3756.153804] R13: 0000000000000000 R14: ffff880035f67070 R15: 0000000000c00000
[ 3756.153836] FS:  0000000000000000(0000) GS:ffff880237cc0000(0000) knlGS:0000000000000000
[ 3756.153871] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 3756.153897] CR2: 00000000000001f0 CR3: 0000000001c08000 CR4: 00000000000006e0
[ 3756.153929] Stack:
[ 3756.153940]  ffff880200000000 ffff880237cd2940 ffff880431f68c00 0000000000000000
[ 3756.153979]  0000000000c00000 ffff880035f67070 0000000000000000 ffff88023464be20
[ 3756.154016]  ffffffffa01e5404 ffff880431f68c80 ffff880234482240 ffff8802378a1800
[ 3756.154054] Call Trace:
[ 3756.154081]  [<ffffffffa01e5404>] btrfs_async_reclaim_metadata_space+0xb4/0x210 [btrfs]
[ 3756.154119]  [<ffffffff8109158e>] process_one_work+0x19e/0x3d0
[ 3756.154146]  [<ffffffff8109180e>] worker_thread+0x4e/0x450
[ 3756.154174]  [<ffffffff816914c9>] ? __schedule+0x2b9/0x930
[ 3756.154199]  [<ffffffff810917c0>] ? process_one_work+0x3d0/0x3d0
[ 3756.154227]  [<ffffffff810917c0>] ? process_one_work+0x3d0/0x3d0
[ 3756.154255]  [<ffffffff81096e59>] kthread+0xc9/0xe0
[ 3756.154279]  [<ffffffff81096d90>] ? kthread_worker_fn+0x160/0x160
[ 3756.154307]  [<ffffffff816956cf>] ret_from_fork+0x3f/0x70
[ 3756.154333]  [<ffffffff81096d90>] ? kthread_worker_fn+0x160/0x160
[ 3756.154361] Code: a5 66 0f 1f 84 00 00 00 00 00 66 66 66 66 90 55 48 89 e5 41 57 41 56 41 55 41 54 49 89 f4 53 31 f6 49 89 fd 49 89 d7 48 83 ec 10 
<4c> 8b b7 f0 01 00 00 89 4d cc 49 3b 7e 30 40 0f 95 c6 48 8d 74
[ 3756.156802] RIP  [<ffffffffa01d9c21>] can_overcommit+0x21/0xf0 [btrfs]
[ 3756.157995]  RSP <ffff88023464bda8>
[ 3756.159162] CR2: 00000000000001f0
============================================================

fs_info->fs_root is referred in btrfs_async_reclaim_metadata_space()
when mount kicked kworker(btrfs_async_reclaim_metadata_space).

But at this time, fs_info->fs_root had not been initialized yet,
so NULL pointer passed to btrfs_calc_reclaim_metadata_size().

============================================================
PID: 3045   TASK: ffff8800bb06b000  CPU: 2   COMMAND: "mount"
    [exception RIP: queued_spin_lock_slowpath+350]
    RIP: ffffffff810be2de  RSP: ffff8800b9fdb738  RFLAGS: 00000202
    RAX: 0000000000000101  RBX: ffff880431f68c00  RCX: 0000000000000001
    RDX: 0000000000000101  RSI: 0000000000000001  RDI: ffff880431f68c00
    RBP: ffff8800b9fdb738   R8: 0000000000000101   R9: 0000000000000000
    R10: 0000000000004000  R11: 0000000000018e58  R12: 0000000000000001
    R13: ffff8800b9fdb7c0  R14: ffff8800bb06b000  R15: 0000000000000001
    CS: 0010  SS: 0018
 #0 [ffff8800b9fdb740] _raw_spin_lock at ffffffff81694ff0
 #1 [ffff8800b9fdb750] reserve_metadata_bytes at ffffffffa01e55cc [btrfs]
 #2 [ffff8800b9fdb800] btrfs_block_rsv_add at ffffffffa01e5a93 [btrfs]
 #3 [ffff8800b9fdb828] btrfs_truncate_inode_items at ffffffffa0202779 [btrfs]
 #4 [ffff8800b9fdb920] btrfs_evict_inode at ffffffffa02040ec [btrfs]
 #5 [ffff8800b9fdb990] evict at ffffffff811ed6ea
 #6 [ffff8800b9fdb9b8] iput at ffffffff811ed996
 #7 [ffff8800b9fdb9e8] btrfs_orphan_cleanup at ffffffffa0204c57 [btrfs]
 #8 [ffff8800b9fdba60] btrfs_recover_relocation at ffffffffa0247a8e [btrfs]
 #9 [ffff8800b9fdbaf0] open_ctree at ffffffffa01f576b [btrfs]
#10 [ffff8800b9fdbbc8] btrfs_mount at ffffffffa01cc6a9 [btrfs]
#11 [ffff8800b9fdbc90] mount_fs at ffffffff811d7ab8
#12 [ffff8800b9fdbcd8] vfs_kern_mount at ffffffff811f1be7
#13 [ffff8800b9fdbd10] btrfs_mount at ffffffffa01cbf57 [btrfs]
#14 [ffff8800b9fdbdd8] mount_fs at ffffffff811d7ab8
#15 [ffff8800b9fdbe20] vfs_kern_mount at ffffffff811f1be7
#16 [ffff8800b9fdbe58] do_mount at ffffffff811f3f8d
#17 [ffff8800b9fdbf08] sys_mount at ffffffff811f4e1c
#18 [ffff8800b9fdbf50] entry_SYSCALL_64_fastpath at ffffffff8169536e
    RIP: 00007f6250fc733a  RSP: 00007ffdcd40ba88  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 00007f6251b2f42a  RCX: 00007f6250fc733a
    RDX: 0000564b58511070  RSI: 0000564b5850e290  RDI: 0000564b5850e270
    RBP: 0000564b5850e150   R8: 0000000000000000   R9: 0000000000000014
    R10: 00000000c0ed0000  R11: 0000000000000246  R12: 00007f6251d3f1dc
    R13: 00007ffdcd40bd88  R14: 0000000000000000  R15: 00000000ffffffff
    ORIG_RAX: 00000000000000a5  CS: 0033  SS: 002b
============================================================

Therefore, check of whether fs_info->fs_root is NULL is added to
btrfs_async_reclaim_metadata_space().

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
---
 fs/btrfs/extent-tree.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 601d7d4..a2621e3 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4694,6 +4694,9 @@ static void btrfs_async_reclaim_metadata_space(struct work_struct *work)
 	fs_info = container_of(work, struct btrfs_fs_info, async_reclaim_work);
 	space_info = __find_space_info(fs_info, BTRFS_BLOCK_GROUP_METADATA);
 
+	if (!fs_info->fs_root)
+		return;
+
 	to_reclaim = btrfs_calc_reclaim_metadata_size(fs_info->fs_root,
 						      space_info);
 	if (!to_reclaim)
