Re: [PATCH 20/27] mm, vmscan: Update classzone_idx if buffer_heads_over_limit

[Michal Hocko]

On Tue 21-06-16 15:15:59, Mel Gorman wrote:
> If buffer heads are over the limit then the direct reclaim gfp_mask
> is promoted to __GFP_HIGHMEM so that lowmem is indirectly freed. With
> node-based reclaim, it is also required that the classzone_idx be updated
> or the pages will be skipped.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/vmscan.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1f7c1262c0a3..f4c759b3581b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2571,8 +2571,10 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc,
>  	 * highmem pages could be pinning lowmem pages storing buffer_heads
>  	 */
>  	orig_mask = sc->gfp_mask;
> -	if (buffer_heads_over_limit)
> +	if (buffer_heads_over_limit) {
>  		sc->gfp_mask |= __GFP_HIGHMEM;
> +		classzone_idx = gfp_zone(sc->gfp_mask);
> +	}
>  
>  	for_each_zone_zonelist_nodemask(zone, z, zonelist,
>  					gfp_zone(sc->gfp_mask), sc->nodemask) {

The code that follows that however does update classzone_idx
		/*
		 * 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--;
		}

so if we start with a zone which doesn't have the highmem zone we will
not see highmems of other zone AFAIU. I guess this would be unlikely
because highmem systems will be UMA in most cases but the code looks a
bit confusing.

-- 
Michal Hocko
SUSE Labs