On 2019/07/01 22:48, Michal Hocko wrote: > On Mon 01-07-19 22:38:58, Tetsuo Handa wrote: >> On 2019/07/01 22:17, Michal Hocko wrote: >>> On Mon 01-07-19 22:04:22, Tetsuo Handa wrote: >>>> But I realized that this patch was too optimistic. We need to wait for mm-less >>>> threads until MMF_OOM_SKIP is set if the process was already an OOM victim. >>> >>> If the process is an oom victim then _all_ threads are so as well >>> because that is the address space property. And we already do check that >>> before reaching oom_badness IIRC. So what is the actual problem you are >>> trying to solve here? >> >> I'm talking about behavioral change after tsk became an OOM victim. >> >> If tsk->signal->oom_mm != NULL, we have to wait for MMF_OOM_SKIP even if >> tsk->mm == NULL. Otherwise, the OOM killer selects next OOM victim as soon as >> oom_unkillable_task() returned true because has_intersects_mems_allowed() returned >> false because mempolicy_nodemask_intersects() returned false because all thread's >> mm became NULL (despite tsk->signal->oom_mm != NULL). > > OK, I finally got your point. It was not clear that you are referring to > the code _after_ the patch you are proposing. You are indeed right that > this would have a side effect that an additional victim could be > selected even though the current process hasn't terminated yet. Sigh, > another example how the whole thing is subtle so I retract my Ack and > request a real life example of where this matters before we think about > a proper fix and make the code even more complex. > Instead of checking for mm != NULL, can we move mpol_put_task_policy() from do_exit() to __put_task_struct() ? That change will (if it is safe to do) prevent exited threads from setting mempolicy = NULL (and confusing mempolicy_nodemask_intersects() due to mempolicy == NULL).
