Re: [PATCH 1/9] mm, page_alloc: rip out ZONELIST_ORDER_ZONE

On 07/14/2017 09:59 AM, Michal Hocko wrote:

From: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

Supporting zone ordered zonelists costs us just a lot of code while
the usefulness is arguable if existent at all. Mel has already made
node ordering default on 64b systems. 32b systems are still using
ZONELIST_ORDER_ZONE because it is considered better to fallback to
a different NUMA node rather than consume precious lowmem zones.

This argument is, however, weaken by the fact that the memory reclaim
has been reworked to be node rather than zone oriented. This means
that lowmem requests have to skip over all highmem pages on LRUs already
and so zone ordering doesn't save the reclaim time much. So the only
advantage of the zone ordering is under a light memory pressure when
highmem requests do not ever hit into lowmem zones and the lowmem
pressure doesn't need to reclaim.

Considering that 32b NUMA systems are rather suboptimal already and
it is generally advisable to use 64b kernel on such a HW I believe we
should rather care about the code maintainability and just get rid of
ZONELIST_ORDER_ZONE altogether. Keep systcl in place and warn if
somebody tries to set zone ordering either from kernel command line
or the sysctl.

Cc: <redacted>
Signed-off-by: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>

Found some leftovers to cleanup:

include/linux/mmzone.h:
extern char numa_zonelist_order[];
#define NUMA_ZONELIST_ORDER_LEN 16      /* string buffer size */

Also update docs?
Documentation/sysctl/vm.txt:zone.  Specify "[Zz]one" for zone order.
Documentation/admin-guide/kernel-parameters.txt:
numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
Documentation/vm/numa:a default zonelist order based on the sizes of the
various zone types relative
Documentation/vm/numa:default zonelist order may be overridden using the
numa_zonelist_order kernel

Otherwise,
Acked-by: Vlastimil Babka <redacted>