linux-kernel - Re: [PATCH v2 1/2] mm/page_alloc: use ac->high_zoneidx for classzone

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Date:   Thu, 19 Mar 2020 13:29:19 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>, kernel-team@....com,
        Ye Xiaolong <xiaolong.ye@...el.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v2 1/2] mm/page_alloc: use ac->high_zoneidx for
 classzone_idx

On 3/18/20 4:32 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> Currently, we use the zone index of preferred_zone which represents
> the best matching zone for allocation, as classzone_idx. It has a problem
> on NUMA system with ZONE_MOVABLE.
> 
> In NUMA system, it can be possible that each node has different populated
> zones. For example, node 0 could have DMA/DMA32/NORMAL/MOVABLE zone and
> node 1 could have only NORMAL zone. In this setup, allocation request
> initiated on node 0 and the one on node 1 would have different
> classzone_idx, 3 and 2, respectively, since their preferred_zones are
> different. If they are handled by only their own node, there is no problem.
> However, if they are somtimes handled by the remote node due to memory
> shortage, the problem would happen.
> 
> In the following setup, allocation initiated on node 1 will have some
> precedence than allocation initiated on node 0 when former allocation is
> processed on node 0 due to not enough memory on node 1. They will have
> different lowmem reserve due to their different classzone_idx thus
> an watermark bars are also different.

...

> Reported-by: Ye Xiaolong <xiaolong.ye@...el.com>
> Tested-by: Ye Xiaolong <xiaolong.ye@...el.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>

With the clarifications that David requested,

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!

> ---
>  mm/internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index c39c895..aebaa33 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -119,7 +119,7 @@ struct alloc_context {
>  	bool spread_dirty_pages;
>  };
>  
> -#define ac_classzone_idx(ac) zonelist_zone_idx(ac->preferred_zoneref)
> +#define ac_classzone_idx(ac) (ac->high_zoneidx)
>  
>  /*
>   * Locate the struct page for both the matching buddy in our
>