general protection fault in ext4_mb_initialize_context

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    8834f560 Linux 5.0-rc5
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14c8021f400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f00801d7b7c4fe6
dashboard link: https://syzkaller.appspot.com/bug?extid=629b913164cba57e45ae
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+629b913164cba57e45ae@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.0.0-rc5 #59
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
RIP: 0010:mutex_can_spin_on_owner kernel/locking/mutex.c:578 [inline]
RIP: 0010:mutex_optimistic_spin kernel/locking/mutex.c:622 [inline]
RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:928 [inline]
RIP: 0010:__mutex_lock+0x207/0x1310 kernel/locking/mutex.c:1072
Code: 00 0f 85 bf 0f 00 00 49 8b 45 00 48 83 e0 f8 0f 84 88 09 00 00 48 8d  
78 38 48 ba 00 00 00 00 00 fc ff df 48 89 f9 48 c1 e9 03 <0f> b6 14 11 84  
d2 74 09 80 fa 03 0f 8e 8b 10 00 00 8b 58 38 85 db
kobject: 'loop1' (00000000ab6ceb2c): kobject_uevent_env
RSP: 0018:ffff8880a986eec0 EFLAGS: 00010206
kobject: 'loop1' (00000000ab6ceb2c): fill_kobj_path: path  
= '/devices/virtual/block/loop1'
RAX: 0000000000000fc0 RBX: ffff8880a98601c0 RCX: 00000000000001ff
RDX: dffffc0000000000 RSI: 0000000000000008 RDI: 0000000000000ff8
RBP: ffff8880a986f030 R08: 1ffffd1ffff85995 R09: fffff91ffff85996
R10: fffff91ffff85995 R11: ffffe8ffffc2ccaf R12: fffffbfff14aad54
R13: ffffe8ffffc2cca8 R14: ffff8880a98601c0 R15: 0000000000000fc0
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd7fe09cdb8 CR3: 00000000a57b2000 CR4: 00000000001426f0
DR0: 0000000000000000 DR1: 0000000037568535 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
 ext4_mb_group_or_file fs/ext4/mballoc.c:4210 [inline]
 ext4_mb_initialize_context+0x7e6/0xbc0 fs/ext4/mballoc.c:4253
 ext4_mb_new_blocks+0x5c4/0x3c70 fs/ext4/mballoc.c:4526
 ext4_ext_map_blocks+0x3094/0x4e50 fs/ext4/extents.c:4404
 ext4_map_blocks+0x8ec/0x1a20 fs/ext4/inode.c:636
 mpage_map_one_extent fs/ext4/inode.c:2480 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2533 [inline]
 ext4_writepages+0x1e00/0x3540 fs/ext4/inode.c:2885
 do_writepages+0xfc/0x2a0 mm/page-writeback.c:2335
 __writeback_single_inode+0x11d/0x12f0 fs/fs-writeback.c:1349
 writeback_sb_inodes+0x596/0xed0 fs/fs-writeback.c:1613
 __writeback_inodes_wb+0xc3/0x260 fs/fs-writeback.c:1682
 wb_writeback+0x87f/0xd00 fs/fs-writeback.c:1791
 wb_check_start_all fs/fs-writeback.c:1915 [inline]
 wb_do_writeback fs/fs-writeback.c:1941 [inline]
 wb_workfn+0xae5/0x1190 fs/fs-writeback.c:1975
 process_one_work+0x98e/0x1790 kernel/workqueue.c:2173
 worker_thread+0x98/0xe40 kernel/workqueue.c:2319
 kthread+0x357/0x430 kernel/kthread.c:246
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace 88f5eb75df3cc65e ]---
RIP: 0010:mutex_can_spin_on_owner kernel/locking/mutex.c:578 [inline]
RIP: 0010:mutex_optimistic_spin kernel/locking/mutex.c:622 [inline]
RIP: 0010:__mutex_lock_common kernel/locking/mutex.c:928 [inline]
RIP: 0010:__mutex_lock+0x207/0x1310 kernel/locking/mutex.c:1072
Code: 00 0f 85 bf 0f 00 00 49 8b 45 00 48 83 e0 f8 0f 84 88 09 00 00 48 8d  
78 38 48 ba 00 00 00 00 00 fc ff df 48 89 f9 48 c1 e9 03 <0f> b6 14 11 84  
d2 74 09 80 fa 03 0f 8e 8b 10 00 00 8b 58 38 85 db
RSP: 0018:ffff8880a986eec0 EFLAGS: 00010206
RAX: 0000000000000fc0 RBX: ffff8880a98601c0 RCX: 00000000000001ff
RDX: dffffc0000000000 RSI: 0000000000000008 RDI: 0000000000000ff8
RBP: ffff8880a986f030 R08: 1ffffd1ffff85995 R09: fffff91ffff85996
R10: fffff91ffff85995 R11: ffffe8ffffc2ccaf R12: fffffbfff14aad54
R13: ffffe8ffffc2cca8 R14: ffff8880a98601c0 R15: 0000000000000fc0
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd7fe09cdb8 CR3: 0000000008871000 CR4: 00000000001426f0
DR0: 0000000000000000 DR1: 0000000037568535 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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