Re: [PATCH] mm,migrate: fix establishing demotion target

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 1/28/2022 1:59 PM, Huang Ying wrote:
> In commit ac16ec835314 ("mm: migrate: support multiple target nodes
> demotion"), after the first demotion target node is found, we will
> continue to check the next candidate obtained via
> find_next_best_node().  This is to find all demotion target nodes with
> same NUMA distance.  But one side effect of find_next_best_node() is
> that the candidate node returned will be set in "used" parameter, even
> if the candidate node isn't passed in the following NUMA distance
> checking, the candidate node will not be used as demotion target node
> for the following nodes.  For example, for system as follows,
> 
> node distances:
> node   0   1   2   3
>    0:  10  21  17  28
>    1:  21  10  28  17
>    2:  17  28  10  28
>    3:  28  17  28  10
> 
> when we establish demotion target node for node 0, in the first round
> node 2 is added to the demotion target node set.  Then in the second
> round, node 3 is checked and failed because distance(0, 3) >
> distance(0, 2).  But node 3 is set in "used" nodemask too.  When we
> establish demotion target node for node 1, there is no available node.
> This is wrong, node 3 should be set as the demotion target of node 1.
> 
> To fix this, if the candidate node is failed to pass the distance
> checking, it will be cleared in "used" nodemask.  So that it can be
> used for the following node.
> 
> The bug can be reproduced and fixed with this patch on a 2 socket
> server machine with DRAM and PMEM.
> 
> Fixes: ac16ec835314 ("mm: migrate: support multiple target nodes demotion")
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: zhongjiang-ali <zhongjiang-ali@linux.alibaba.com>
> Cc: Xunlei Pang <xlpang@linux.alibaba.com>
> ---

Good catch. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

>   mm/migrate.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c7da064b4781..e8a6933af68d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -3082,18 +3082,21 @@ static int establish_migrate_target(int node, nodemask_t *used,
>   	if (best_distance != -1) {
>   		val = node_distance(node, migration_target);
>   		if (val > best_distance)
> -			return NUMA_NO_NODE;
> +			goto out_clear;
>   	}
>   
>   	index = nd->nr;
>   	if (WARN_ONCE(index >= DEMOTION_TARGET_NODES,
>   		      "Exceeds maximum demotion target nodes\n"))
> -		return NUMA_NO_NODE;
> +		goto out_clear;
>   
>   	nd->nodes[index] = migration_target;
>   	nd->nr++;
>   
>   	return migration_target;
> +out_clear:
> +	node_clear(migration_target, *used);
> +	return NUMA_NO_NODE;
>   }
>   
>   /*