Hi Michal
The null pointer is possible when calling the dump_header, this bug was
detected by LKP. Below is the context 3 months ago.


On Mon 30-07-18 19:05:50, David Rientjes wrote:
> On Mon, 30 Jul 2018, Michal Hocko wrote:
>
> > On Mon 30-07-18 17:03:20, kernel test robot wrote:
> > [...]
> > > [    9.034310] BUG: KASAN: null-ptr-deref in dump_header+0x10c/0x448
> >
> > Could you faddr2line on the offset please?
> >
>
> It's possible that p is NULL when calling dump_header().  In this case we
> do not want to print any line concerning a victim because no oom kill has
> occurred.

> You are right. I have missed those.

> This code shouldn't be part of dump_header(), which is called from
> multiple contexts even when an oom kill has not occurred, and is
> ratelimited.  The single line output should be the canonical way that
> userspace parses the log for oom victims, we can't ratelimit it.
>
> The following would be a fix patch, but it will be broken if the cgroup
> aware oom killer is removed from -mm so that the oom_group stuff can be
> merged.

> cgroup aware oom killer is going to be replaced by a new implementation
> IIUC so the fix should be based on the yuzhoujian patch. Ideally to be
> resubmitted.

> I would just suggest adding it into a function
> dump_oom_summary(struct oom_control *oc, struct task_struct *p)

> yuzhoujian could you take care of that please?

I followed David's tip and call the new func dump_oom_summary in the
oom_kill_process.

> It's possible that p is NULL when calling dump_header().  In this case we
> do not want to print any line concerning a victim because no oom kill has
>occurred.

> This code shouldn't be part of dump_header(), which is called from
> multiple contexts even when an oom kill has not occurred, and is
> ratelimited.  The single line output should be the canonical way that
> userspace parses the log for oom victims, we can't ratelimit it.

> The following would be a fix patch, but it will be broken if the cgroup
> aware oom killer is removed from -mm so that the oom_group stuff can be
> merged.