Re: mm, oom_reaper: How to handle race with oom_killer_disable() ?

[Tetsuo Handa]

Michal Hocko wrote:
> On Wed 22-06-16 00:32:29, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> [...]
> > > Hmm, what about the following instead. It is rather a workaround than a
> > > full flaged fix but it seems much more easier and shouldn't introduce
> > > new issues.
> > 
> > Yes, I think that will work. But I think below patch (marking signal_struct
> > to ignore TIF_MEMDIE instead of clearing TIF_MEMDIE from task_struct) on top of
> > current linux.git will implement no-lockup requirement. No race is possible unlike
> > "[PATCH 10/10] mm, oom: hide mm which is shared with kthread or global init".
> 
> Not really. Because without the exit_oom_victim from oom_reaper you have
> no guarantee that the oom_killer_disable will ever return. I have
> mentioned that in the changelog. There is simply no guarantee the oom
> victim will ever reach exit_mm->exit_oom_victim.

Why? Since any allocation after setting oom_killer_disabled = true will be
forced to fail, nobody will be blocked on waiting for memory allocation. Thus,
the TIF_MEMDIE tasks will eventually reach exit_mm->exit_oom_victim, won't it?

The only possibility that the TIF_MEMDIE tasks won't reach exit_mm->exit_oom_victim
is __GFP_NOFAIL allocations failing to make forward progress even after
ALLOC_NO_WATERMARKS is used. But that is a different problem which I think
we can call panic() when __GFP_NOFAIL allocations failed after setting
oom_killer_disabled = true.