Am 13.12.2017 um 04:58 schrieb Yan, Zheng:

We recently got an Oops report:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: jbd2__journal_start+0x38/0x1a2
[...]
Call Trace:
  ext4_page_mkwrite+0x307/0x52b
  _ext4_get_block+0xd8/0xd8
  do_page_mkwrite+0x6e/0xd8
  handle_mm_fault+0x686/0xf9b
  mntput_no_expire+0x1f/0x21e
  __do_page_fault+0x21d/0x465
  dput+0x4a/0x2f7
  page_fault+0x22/0x30
  copy_user_generic_string+0x2c/0x40
  copy_page_to_iter+0x8c/0x2b8
  generic_file_read_iter+0x26e/0x845
  timerqueue_del+0x31/0x90
  ceph_read_iter+0x697/0xa33 [ceph]
  hrtimer_cancel+0x23/0x41
  futex_wait+0x1c8/0x24d
  get_futex_key+0x32c/0x39a
  __vfs_read+0xe0/0x130
  vfs_read.part.1+0x6c/0x123
  handle_mm_fault+0x831/0xf9b
  __fget+0x7e/0xbf
  SyS_read+0x4d/0xb5

The reason is that page fault can happen when one filesystem copies
data from/to userspace, the filesystem may set current->journal_info.
If the userspace memory is mapped to a file on another filesystem,
the later filesystem may also want to use current->journal_info.

Signed-off-by: "Yan, Zheng" <redacted>

Reported-and-tested-by: Amon Ott <redacted>

Thanks a lot for the patch! I have ported your patch to 4.9.68, tested
and the bug seems fixed now.

Amon Ott
-- 
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