mm: page_alloc: exclude unreclaimable allocations from zone fairness policy

mm: page_alloc: exclude unreclaimable allocations from zone fairness policy

Dave Hansen noted a regression in a microbenchmark that loops around
open() and close() on an 8-node NUMA machine and bisected it down to
81c0a2bb515f ("mm: page_alloc: fair zone allocator policy").  That change
forces the slab allocations of the file descriptor to spread out to all 8
nodes, causing remote references in the page allocator and slab.

The round-robin policy is only there to provide fairness among memory
allocations that are reclaimed involuntarily based on pressure in each
zone.  It does not make sense to apply it to unreclaimable kernel
allocations that are freed manually, in this case instantly after the
allocation, and incur the remote reference costs twice for no reason.

Only round-robin allocations that are usually freed through page reclaim
or slab shrinking.

Bisected by Dave Hansen.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>