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

[Michal Hocko <mhocko@kernel.org>]

On Fri 14-07-17 10:36:50, Mel Gorman wrote:
> On Fri, Jul 14, 2017 at 09:59:58AM +0200, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@suse.com>
> > 
> > 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@vger.kernel.org>
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > 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.

Ohh, right! That is indeed strange. Then I guess it would be probably
better to simply return Node to make it clear what the default is. What
do you think?

> 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@suse.de>

Thanks!
-- 
Michal Hocko
SUSE Labs