Re: [PATCH 15/27] mm, page_alloc: Consider dirtyable memory in terms of nodes

[Mel Gorman]

On Wed, Jun 22, 2016 at 04:27:57PM +0200, Michal Hocko wrote:
> > which can use it (e.g. vmalloc). I understand how this is both an
> > inherent problem of 32b with a larger high:low ratio and why it is hard
> > to at least pretend we can cope with it with node based approach but we
> > should at least document it.
> > 
> > I workaround would be to enable highmem_dirtyable_memory which can lead
> > to premature OOM killer for some workloads AFAIR.
> [...]
> > >  static unsigned long highmem_dirtyable_memory(unsigned long total)
> > >  {
> > >  #ifdef CONFIG_HIGHMEM
> > > -	int node;
> > >  	unsigned long x = 0;
> > > -	int i;
> > > -
> > > -	for_each_node_state(node, N_HIGH_MEMORY) {
> > > -		for (i = 0; i < MAX_NR_ZONES; i++) {
> > > -			struct zone *z = &NODE_DATA(node)->node_zones[i];
> > >  
> > > -			if (is_highmem(z))
> > > -				x += zone_dirtyable_memory(z);
> > > -		}
> > > -	}
> 
> Hmm, I have just noticed that we have NR_ZONE_LRU_ANON resp.
> NR_ZONE_LRU_FILE so we can estimate the amount of highmem contribution
> to the global counters by the following or similar:
> 
> 	for_each_node_state(node, N_HIGH_MEMORY) {
> 		for (i = 0; i < MAX_NR_ZONES; i++) {
> 			struct zone *z = &NODE_DATA(node)->node_zones[i];
> 
> 			if (!is_highmem(z))
> 				continue;
> 
> 			x += zone_page_state(z, NR_FREE_PAGES) + zone_page_state(z, NR_ZONE_LRU_FILE) - high_wmark_pages(zone);
> 		}
> 
> high wmark reduction would be to emulate the reserve. What do you think?

Agreed with minor modifications. Went with this

        for_each_node_state(node, N_HIGH_MEMORY) {
                for (i = ZONE_NORMAL + 1; i < MAX_NR_ZONES; i++) {
                        struct zone *z;

                        if (!is_highmem_idx(z))
                                continue;

                        z = &NODE_DATA(node)->node_zones[i];
                        x += zone_page_state(z, NR_FREE_PAGES) +
                                zone_page_state(z, NR_ZONE_LRU_FILE) -
                                high_wmark_pages(zone);
                }
        }

-- 
Mel Gorman
SUSE Labs