Re: kernel BUG at mm/shmem.c:LINE!

[Matthew Wilcox]

On Fri, Jul 06, 2018 at 06:19:02PM -0700, syzbot wrote:
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+b8e0dfee3fd8c9012771@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> raw: 02fffc0000001028 ffffea0007011dc8 ffffea0007058b48 ffff8801a7576ab8
> raw: 000000000000016e ffff8801a7588930 00000003ffffffff ffff8801d9a44c80
> page dumped because: VM_BUG_ON_PAGE(page_to_pgoff(page) != index)
> page->mem_cgroup:ffff8801d9a44c80
> ------------[ cut here ]------------
> kernel BUG at mm/shmem.c:815!
> invalid opcode: 0000 [#1] SMP KASAN
> CPU: 0 PID: 4429 Comm: syz-executor697 Not tainted 4.18.0-rc3-next-20180706+
> #1
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:shmem_undo_range+0xdaa/0x29a0 mm/shmem.c:815

Pretty sure this one's mine.  At least I spotted a codepath earlier
today which could lead to it.  I'll fix it in the morning.