mm, oom: give __GFP_NOFAIL allocations access to memory reserves

__GFP_NOFAIL is a big hammer used to ensure that the allocation request
can never fail.  This is a strong requirement and as such it also
deserves a special treatment when the system is OOM.  The primary
problem here is that the allocation request might have come with some
locks held and the oom victim might be blocked on the same locks.  This
is basically an OOM deadlock situation.

This patch tries to reduce the risk of such a deadlocks by giving
__GFP_NOFAIL allocations a special treatment and let them dive into
memory reserves after oom killer invocation.  This should help them to
make a progress and release resources they are holding.  The OOM victim
should compensate for the reserves consumption.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Suggested-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>