Re: [PATCH v4 11/11] mm/memory_hotplug: remove a wrapper for alloc_migration_target()

[Vlastimil Babka]

On 7/7/20 9:44 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> To calculate the correct node to migrate the page for hotplug, we need
> to check node id of the page. Wrapper for alloc_migration_target() exists
> for this purpose.
> 
> However, Vlastimil informs that all migration source pages come from
> a single node. In this case, we don't need to check the node id for each
> page and we don't need to re-set the target nodemask for each page by
> using the wrapper. Set up the migration_target_control once and use it for
> all pages.
> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks! Nitpick below.

> @@ -1345,9 +1324,28 @@ do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  		put_page(page);
>  	}
>  	if (!list_empty(&source)) {
> -		/* Allocate a new page from the nearest neighbor node */
> -		ret = migrate_pages(&source, new_node_page, NULL, 0,
> -					MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
> +		nodemask_t nmask = node_states[N_MEMORY];
> +		struct migration_target_control mtc = {
> +			.nmask = &nmask,
> +			.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
> +		};
> +
> +		/*
> +		 * We have checked that migration range is on a single zone so
> +		 * we can use the nid of the first page to all the others.
> +		 */
> +		mtc.nid = page_to_nid(list_first_entry(&source, struct page, lru));
> +
> +		/*
> +		 * try to allocate from a different node but reuse this node
> +		 * if there are no other online nodes to be used (e.g. we are
> +		 * offlining a part of the only existing node)
> +		 */
> +		node_clear(mtc.nid, *mtc.nmask);
> +		if (nodes_empty(*mtc.nmask))
> +			node_set(mtc.nid, *mtc.nmask);

You could have kept using 'nmask' instead of '*mtc.nmask'. Actually that applies
to patch 6 too, for less churn.

> +		ret = migrate_pages(&source, alloc_migration_target, NULL,
> +			(unsigned long)&mtc, MIGRATE_SYNC, MR_MEMORY_HOTPLUG);
>  		if (ret) {
>  			list_for_each_entry(page, &source, lru) {
>  				pr_warn("migrating pfn %lx failed ret:%d ",
>