EXT4-fs (sda1): re-mounted. Opts: dax,
EXT4-fs (sda1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
EXT4-fs: 380 callbacks suppressed
EXT4-fs (sda1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
EXT4-fs (sda1): warning: refusing change of dax flag with busy inodes while remounting
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
EXT4-fs (sda1): re-mounted. Opts: dax,
IP:           (null)
PGD 8ea1b067 P4D 8ea1b067 PUD 9beaa067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 1 PID: 14920 Comm: syz-executor.3 Not tainted 4.16.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:          (null)
RSP: 0018:ffff88009da97968 EFLAGS: 00010246
RAX: ffffffff86f738e0 RBX: 0000000000000001 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffff880076df28f8 RDI: ffff88008aaf3500
RBP: ffff88009da97aa0 R08: 0000000000000000 R09: ffff88009da979f8
R10: ffff880072548a70 R11: 0000000000000001 R12: 0000000000000000
R13: ffff88009da97a78 R14: ffff880076df28f8 R15: ffff880072548140
FS:  00007f133a935700(0000) GS:ffff8800aed00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000098ff4000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __generic_file_write_iter+0x201/0x580 mm/filemap.c:3263
 ext4_file_write_iter+0x295/0xfd0 fs/ext4/file.c:266
 call_write_iter include/linux/fs.h:1781 [inline]
 new_sync_write fs/read_write.c:469 [inline]
 __vfs_write+0x42d/0x860 fs/read_write.c:482
 vfs_write+0x150/0x4f0 fs/read_write.c:544
 SYSC_write fs/read_write.c:589 [inline]
 SyS_write+0xcd/0x1b0 fs/read_write.c:581
 do_syscall_64+0x1c7/0x690 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45c889
RSP: 002b:00007f133a934c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f133a9356d4 RCX: 000000000045c889
RDX: 0000000000000001 RSI: 0000000020000080 RDI: 0000000000000003
RBP: 000000000076bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000cdc R14: 00000000004cf042 R15: 000000000076bf0c
Code:  Bad RIP value.
RIP:           (null) RSP: ffff88009da97968
CR2: 0000000000000000
EXT4-fs (sda1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
---[ end trace 4923fd02a42209f8 ]---
EXT4-fs (sda1): warning: refusing change of dax flag with busy inodes while remounting