Re: crash during oom reaper

[Vegard Nossum]

On 12/16/2016 03:32 PM, Michal Hocko wrote:
> On Fri 16-12-16 15:25:27, Vegard Nossum wrote:
> > On 12/16/2016 03:00 PM, Michal Hocko wrote:
> > > On Fri 16-12-16 14:14:17, Vegard Nossum wrote:
> > > [...]
> > > > Out of memory: Kill process 1650 (trinity-main) score 90 or sacrifice child
> > > > Killed process 1724 (trinity-c14) total-vm:37280kB, anon-rss:236kB,
> > > > file-rss:112kB, shmem-rss:112kB
> > > > BUG: unable to handle kernel NULL pointer dereference at 00000000000001e8
> > > > IP: [<ffffffff8126b1c0>] copy_process.part.41+0x2150/0x5580
> > > > PGD c001067 PUD c000067
> > > > PMD 0
> > > > Oops: 0002 [#1] PREEMPT SMP KASAN
> > > > Dumping ftrace buffer:
> > > >    (ftrace buffer empty)
> > > > CPU: 28 PID: 1650 Comm: trinity-main Not tainted 4.9.0-rc6+ #317
> > >
> > > Hmm, so this was the oom victim initially but we have decided to kill
> > > its child 1724 instead.
> > >
> > > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> > > > Ubuntu-1.8.2-1ubuntu1 04/01/2014
> > > > task: ffff88000f9bc440 task.stack: ffff88000c778000
> > > > RIP: 0010:[<ffffffff8126b1c0>]  [<ffffffff8126b1c0>]
> > > > copy_process.part.41+0x2150/0x5580
> > >
> > > Could you match this to the kernel source please?
> >
> > kernel/fork.c:629 dup_mmap()
>
> Ok, so this is before the child is made visible so the oom reaper
> couldn't have seen it.
>
> > it's atomic_dec(&inode->i_writecount), it matches up with
> > file_inode(file) == NULL:
> >
> > (gdb) p &((struct inode *)0)->i_writecount
> > $1 = (atomic_t *) 0x1e8 <irq_stack_union+488>
>
> is this a p9 inode?

When I looked at this before it always crashed in this spot for the very
first VMA in the mm (which happens to be the exe, which is on a 9p root fs).

I added a trace_printk() to dup_mmap() to print inode->i_sb->s_type and
the last thing I see for a new crash in the same place is:

trinity--9280   28.... 136345090us : copy_process.part.41: ffffffff8485ec40
---------------------------------
CPU: 0 PID: 9302 Comm: trinity-c0 Not tainted 4.9.0-rc8+ #332
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
Ubuntu-1.8.2-1ubuntu1 04/01/2014
task: ffff880000070000 task.stack: ffff8800099e0000
RIP: 0010:[<ffffffff8126c7c9>]  [<ffffffff8126c7c9>] 
copy_process.part.41+0x22c9/0x55b0

As you can see, the addresses match:

(gdb) p &v9fs_fs_type
$1 = (struct file_system_type *) 0xffffffff8485ec40 <v9fs_fs_type>

So I think we can safely say that yes, it's a p9 inode.


Vegard