BUG: unable to handle kernel NULL pointer dereference in __generic_file_write_iter

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    4aa9fc2a435a Revert "mm, memory_hotplug: initialize struct..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1101382f400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4fceea9e2d99ac20
dashboard link: https://syzkaller.appspot.com/bug?extid=ca95b2b7aef9e7cbd6ab
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+ca95b2b7aef9e7cbd6ab@xxxxxxxxxxxxxxxxxxxxxxxxx

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
#PF error: [INSTR]
PGD a7ea0067 P4D a7ea0067 PUD 81535067 PMD 0
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 15924 Comm: syz-executor0 Not tainted 5.0.0-rc4+ #50
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff88804c3d7858 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffffff885aeb60 RCX: 000000000000005b
RDX: 0000000000000000 RSI: ffff88807ec22930 RDI: ffff8880a59bdcc0
RBP: ffff88804c3d79b8 R08: 0000000000000000 R09: ffff88804c3d7910
R10: ffff8880835ca200 R11: 0000000000000000 R12: 000000000000005b
R13: ffff88804c3d7c98 R14: dffffc0000000000 R15: 0000000000000000
FS:  00007f3456db4700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000000814ac000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __generic_file_write_iter+0x25e/0x630 mm/filemap.c:3333
 ext4_file_write_iter+0x37a/0x1410 fs/ext4/file.c:266
 call_write_iter include/linux/fs.h:1862 [inline]
 new_sync_write fs/read_write.c:474 [inline]
 __vfs_write+0x764/0xb40 fs/read_write.c:487
 vfs_write+0x20c/0x580 fs/read_write.c:549
 ksys_write+0x105/0x260 fs/read_write.c:598
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write fs/read_write.c:607 [inline]
 __x64_sys_write+0x73/0xb0 fs/read_write.c:607
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x458089
Code: 6d b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f3456db3c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000458089
RDX: 000000000000005b RSI: 0000000020000240 RDI: 0000000000000003
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f3456db46d4
R13: 00000000004c7450 R14: 00000000004dce68 R15: 00000000ffffffff
Modules linked in:
CR2: 0000000000000000
---[ end trace 5cac9d2c75a59916 ]---
kobject: 'loop5' (000000004426a409): kobject_uevent_env
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff88804c3d7858 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffffff885aeb60 RCX: 000000000000005b
kobject: 'loop5' (000000004426a409): fill_kobj_path: path  
= '/devices/virtual/block/loop5'
RDX: 0000000000000000 RSI: ffff88807ec22930 RDI: ffff8880a59bdcc0
kobject: 'loop2' (00000000b82e0c58): kobject_uevent_env
kobject: 'loop2' (00000000b82e0c58): fill_kobj_path: path  
= '/devices/virtual/block/loop2'
RBP: ffff88804c3d79b8 R08: 0000000000000000 R09: ffff88804c3d7910
R10: ffff8880835ca200 R11: 0000000000000000 R12: 000000000000005b
R13: ffff88804c3d7c98 R14: dffffc0000000000 R15: 0000000000000000
kobject: 'loop5' (000000004426a409): kobject_uevent_env
FS:  00007f3456db4700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
kobject: 'loop5' (000000004426a409): fill_kobj_path: path  
= '/devices/virtual/block/loop5'
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000022029a0 CR3: 00000000814ac000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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