mm, oom: allow exiting threads to have access to memory reserves

Exiting threads, those with PF_EXITING set, can pagefault and require
memory before they can make forward progress.  This happens, for
instance,
when a process must fault task->robust_list, a userspace structure,
before
detaching its memory.

These threads also aren't guaranteed to get access to memory reserves
unless oom killed or killed from userspace.  The oom killer won't grant
memory reserves if other threads are also exiting other than current and
stalling at the same point.  This prevents needlessly killing processes
when others are already exiting.

Instead of special casing all the possible situations between PF_EXITING
getting set and a thread detaching its mm where it may allocate memory,
which probably wouldn't get updated when a change is made to the exit
path, the solution is to give all exiting threads access to memory
reserves if they call the oom killer.  This allows them to quickly
allocate, detach its mm, and free the memory it represents.

Summary of Luigi's bug report:

: He had an oom condition where threads were faulting on
task->robust_list
: and repeatedly called the oom killer but it would defer killing a
thread
: because it saw other PF_EXITING threads.  This can happen anytime we
need
: to allocate memory after setting PF_EXITING and before detaching our
mm;
: if there are other threads in the same state then the oom killer won't
do
: anything unless one of them happens to be killed from userspace.
:
: So instead of only deferring for PF_EXITING and !task->robust_list,
it's
: better to just give them access to memory reserves to prevent a
potential
: livelock so that any other faults that may be introduced in the future
in
: the exit path don't cause the same problem (and hopefully we don't
allow
: too many of those!).

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Tested-by: Luigi Semenzato <semenzato@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

idea is taken here -
https://github.com/faux123/moto_x/commit/f499bb2248e674490fe52ad49ce26ad17e15dd26