mm, oom: hide mm which is shared with kthread or global init

The only case where the oom_reaper is not triggered for the oom victim
is when it shares the memory with a kernel thread (aka use_mm) or with
the global init.  After "mm, oom: skip vforked tasks from being
selected" the victim cannot be a vforked task of the global init so we
are left with clone(CLONE_VM) (without CLONE_SIGHAND).  use_mm() users
are quite rare as well.

In order to help forward progress for the OOM killer, make sure that
this really rare case will not get in the way - we do this by hiding the
mm from the oom killer by setting MMF_OOM_REAPED flag for it.
oom_scan_process_thread will ignore any TIF_MEMDIE task if it has
MMF_OOM_REAPED flag set to catch these oom victims.

After this patch we should guarantee forward progress for the OOM killer
even when the selected victim is sharing memory with a kernel thread or
global init as long as the victims mm is still alive.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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