BUG: unable to handle kernel NULL pointer dereference in __do_page_cache_readahead

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    3d0e7a9e00fd Merge tag 'md/4.19-rc2' of git://git.kernel.o..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1654ac21400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8f59875069d721b6
dashboard link: https://syzkaller.appspot.com/bug?extid=d47b586c9bc26763ffce
compiler:       gcc (GCC) 8.0.1 20180413 (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+d47b586c9bc26763ffce@xxxxxxxxxxxxxxxxxxxxxxxxx

EXT4-fs (sda1): warning: refusing change of dax flag with busy inodes while  
remounting
EXT4-fs (sda1): re-mounted. Opts: dax,,errors=continue
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
EXT4-fs (sda1): re-mounted. Opts: dax,,errors=continue
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 1cf0bc067 P4D 1cf0bc067 PUD 1c8d95067 PMD 0
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 9112 Comm: syz-executor2 Not tainted 4.19.0-rc2+ #6
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff880184436a28 EFLAGS: 00010246
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: ffffc900062fe000
RDX: 1ffffffff1036431 RSI: ffffea000602f280 RDI: ffff8801c86ce780
RBP: ffff880184436c08 R08: ffff8801bf28a0c0 R09: fffff94000c05e56
R10: fffff94000c05e56 R11: ffffea000602f2b7 R12: ffffea000602f288
R13: ffffea000602f280 R14: 0000000000000000 R15: ffffed0030886d74
FS:  00007f9fe8e5f700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000018aff0000 CR4: 00000000001406e0
Call Trace:
 __do_page_cache_readahead+0x56d/0x980 mm/readahead.c:210
 ra_submit mm/internal.h:66 [inline]
 do_sync_mmap_readahead mm/filemap.c:2444 [inline]
 filemap_fault+0xf4d/0x25f0 mm/filemap.c:2520
 ext4_filemap_fault+0x82/0xad fs/ext4/inode.c:6257
 __do_fault+0x100/0x6b0 mm/memory.c:3240
 do_read_fault mm/memory.c:3652 [inline]
 do_fault mm/memory.c:3752 [inline]
 handle_pte_fault mm/memory.c:3983 [inline]
 __handle_mm_fault+0x3709/0x53e0 mm/memory.c:4107
 handle_mm_fault+0x54f/0xc70 mm/memory.c:4144
 faultin_page mm/gup.c:518 [inline]
 __get_user_pages+0x806/0x1b30 mm/gup.c:718
 populate_vma_page_range+0x2db/0x3d0 mm/gup.c:1222
 __mm_populate+0x286/0x4d0 mm/gup.c:1270
 mm_populate include/linux/mm.h:2307 [inline]
 vm_mmap_pgoff+0x27f/0x2c0 mm/util.c:362
 ksys_mmap_pgoff+0x4da/0x660 mm/mmap.c:1585
 __do_sys_mmap arch/x86/kernel/sys_x86_64.c:100 [inline]
 __se_sys_mmap arch/x86/kernel/sys_x86_64.c:91 [inline]
 __x64_sys_mmap+0xe9/0x1b0 arch/x86/kernel/sys_x86_64.c:91
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457099
Code: fd b4 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 cb b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f9fe8e5ec78 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
RAX: ffffffffffffffda RBX: 00007f9fe8e5f6d4 RCX: 0000000000457099
RDX: 00000000007ffffe RSI: 0000000000600000 RDI: 0000000020000000
RBP: 0000000000930140 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000004002011 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004d3168 R14: 00000000004c8161 R15: 0000000000000001
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
CR2: 0000000000000000
---[ end trace e3fb6a18760358f2 ]---
RIP: 0010:          (null)
Code: Bad RIP value.
RSP: 0018:ffff880184436a28 EFLAGS: 00010246
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: ffffc900062fe000
RDX: 1ffffffff1036431 RSI: ffffea000602f280 RDI: ffff8801c86ce780
RBP: ffff880184436c08 R08: ffff8801bf28a0c0 R09: fffff94000c05e56
R10: fffff94000c05e56 R11: ffffea000602f2b7 R12: ffffea000602f288
R13: ffffea000602f280 R14: 0000000000000000 R15: ffffed0030886d74
FS:  00007f9fe8e5f700(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
kobject: 'loop3' (0000000092ea2d98): kobject_uevent_env
kobject: 'loop3' (0000000092ea2d98): fill_kobj_path: path  
= '/devices/virtual/block/loop3'
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kobject: 'loop7' (00000000102bc430): kobject_uevent_env
CR2: 00000000004d3d08 CR3: 000000018aff0000 CR4: 00000000001406f0
kobject: 'loop7' (00000000102bc430): fill_kobj_path: path  
= '/devices/virtual/block/loop7'
kobject: 'loop1' (00000000bbec18b6): kobject_uevent_env
kobject: 'loop5' (000000006bdd6403): kobject_uevent_env
kobject: 'loop5' (000000006bdd6403): fill_kobj_path: path  
= '/devices/virtual/block/loop5'
kobject: 'loop1' (00000000bbec18b6): fill_kobj_path: path  
= '/devices/virtual/block/loop1'


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