Re: [PATCH 04/27] mm, vmscan: Begin reclaiming pages on a per-node basis

[Vlastimil Babka]

On 06/22/2016 04:04 PM, Vlastimil Babka wrote:
> On 06/21/2016 04:15 PM, Mel Gorman wrote:
> > This patch makes reclaim decisions on a per-node basis. A reclaimer knows
> > what zone is required by the allocation request and skips pages from
> > higher zones. In many cases this will be ok because it's a GFP_HIGHMEM
> > request of some description. On 64-bit, ZONE_DMA32 requests will cause
> > some problems but 32-bit devices on 64-bit platforms are increasingly
> > rare. Historically it would have been a major problem on 32-bit with big
> > Highmem:Lowmem ratios but such configurations are also now rare and even
> > where they exist, they are not encouraged. If it really becomes a problem,
> > it'll manifest as very low reclaim efficiencies.
> >
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>
> [...]
>
> > @@ -2540,14 +2559,14 @@ static inline bool compaction_ready(struct zone *zone, int order, int classzone_
> >   * If a zone is deemed to be full of pinned pages then just give it a light
> >   * scan then give up on it.
> >   */
> > -static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> > +static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc,
> > +		enum zone_type classzone_idx)
> >  {
> >  	struct zoneref *z;
> >  	struct zone *zone;
> >  	unsigned long nr_soft_reclaimed;
> >  	unsigned long nr_soft_scanned;
> >  	gfp_t orig_mask;
> > -	enum zone_type requested_highidx = gfp_zone(sc->gfp_mask);
> >
> >  	/*
> >  	 * If the number of buffer_heads in the machine exceeds the maximum
> > @@ -2560,15 +2579,20 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> >
> >  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
> >  					gfp_zone(sc->gfp_mask), sc->nodemask) {
>
> Using sc->reclaim_idx could be faster/nicer here than gfp_zone()?
> Although after "mm, vmscan: Update classzone_idx if buffer_heads_over_limit"
> there would need to be a variable for the highmem adjusted value - maybe reuse
> "requested_highidx"? Not important though.
>
> > -		enum zone_type classzone_idx;
> > -
> >  		if (!populated_zone(zone))
> >  			continue;
> >
> > -		classzone_idx = requested_highidx;
> > +		/*
> > +		 * Note that reclaim_idx does not change as it is the highest
> > +		 * zone reclaimed from which for empty zones is a no-op but
> > +		 * classzone_idx is used by shrink_node to test if the slabs
> > +		 * should be shrunk on a given node.
> > +		 */
> >  		while (!populated_zone(zone->zone_pgdat->node_zones +
> > -							classzone_idx))
> > +							classzone_idx)) {
> >  			classzone_idx--;
> > +			continue;

Oh and Michal's comment on Patch 20 made me realize that my objection to v6 
about possible underflow of sc->reclaim_idx and classzone_idx seems to still 
apply here for classzone_idx? Updated example: Normal zone allocation. A small 
node 0 without Normal zone will get us classzone_idx == dma32. Node 1 next in 
zonelist won't have dma/dma32 zones so we won't see node_zones + classzone_idx 
populated, and the while loop will lead to underflow of classzone_idx.
I may be missing something, but I don't really see another way around it than 
resetting classzone_idx to sc->reclaim_idx before the while loop.