mm, oom: kill all tasks sharing the mm

Currently oom_kill_process skips both the oom reaper and SIG_KILL if a
process sharing the same mm is unkillable via OOM_ADJUST_MIN.  After "mm,
oom_adj: make sure processes sharing mm have same view of oom_score_adj"
all such processes are sharing the same value so we shouldn't see such a
task at all (oom_badness would rule them out).

We can still encounter oom disabled vforked task which has to be killed as
well if we want to have other tasks sharing the mm reapable because it can
access the memory before doing exec.  Killing such a task should be
acceptable because it is highly unlikely it has done anything useful
because it cannot modify any memory before it calls exec.  An alternative
would be to keep the task alive and skip the oom reaper and risk all the
weird corner cases where the OOM killer cannot make forward progress
because the oom victim hung somewhere on the way to exit.

[rientjes@google.com - drop printk when OOM_SCORE_ADJ_MIN killed task
 the setting is inherently racy and we cannot do much about it without
 introducing locks in hot paths]
Link: http://lkml.kernel.org/r/1466426628-15074-7-git-send-email-mhocko@kernel.org


Signed-off-by: Michal Hocko <mhocko@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>