On Fri, Jul 14, 2017 at 09:59:58AM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> 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: <linux-api@xxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> index 80e4adb4c360..d9f4ea057e74 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4864,40 +4824,22 @@ int numa_zonelist_order_handler(struct ctl_table *table, int write,
> void __user *buffer, size_t *length,
> loff_t *ppos)
> {
> - char saved_string[NUMA_ZONELIST_ORDER_LEN];
> + char *str;
> int ret;
> - static DEFINE_MUTEX(zl_order_mutex);
>
> - mutex_lock(&zl_order_mutex);
> - if (write) {
> - if (strlen((char *)table->data) >= NUMA_ZONELIST_ORDER_LEN) {
> - ret = -EINVAL;
> - goto out;
> - }
> - strcpy(saved_string, (char *)table->data);
> + if (!write) {
> + int len = sizeof("Default");
> + if (copy_to_user(buffer, "Default", len))
> + return -EFAULT;
> + return len;
> }
That should to be "default" because the original code would have the proc
entry display "default" unless it was set at runtime. Pretty weird I
know but it's always possible someone is parsing the original default
and not handling it properly.
Otherwise I think we're way past the point where large memory 32-bit
NUMA machines are a thing so
Acked-by: Mel Gorman <mgorman@xxxxxxx>
--
Mel Gorman
SUSE Labs
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