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Date:   Tue, 7 Jul 2020 13:52:31 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     js1304@gmail.com
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Vlastimil Babka <vbabka@suse.cz>,
        Christoph Hellwig <hch@infradead.org>,
        Roman Gushchin <guro@fb.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v4 11/11] mm/memory_hotplug: remove a wrapper for
 alloc_migration_target()
Message-ID: <20200707115231.GM5913@dhcp22.suse.cz>
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 <1594107889-32228-12-git-send-email-iamjoonsoo.kim@lge.com>
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On Tue 07-07-20 16:44:49, Joonsoo Kim wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> 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.

yes, memory offlining only operates on a single zone. Have a look at
test_pages_in_a_zone().

> 
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memory_hotplug.c | 46 ++++++++++++++++++++++------------------------
>  1 file changed, 22 insertions(+), 24 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 86bc2ad..269e8ca 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1265,27 +1265,6 @@ static int scan_movable_pages(unsigned long start, unsigned long end,
>  	return 0;
>  }
>  
> -static struct page *new_node_page(struct page *page, unsigned long private)
> -{
> -	nodemask_t nmask = node_states[N_MEMORY];
> -	struct migration_target_control mtc = {
> -		.nid = page_to_nid(page),
> -		.nmask = &nmask,
> -		.gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
> -	};
> -
> -	/*
> -	 * 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);
> -
> -	return alloc_migration_target(page, (unsigned long)&mtc);
> -}
> -
>  static int
>  do_migrate_range(unsigned long start_pfn, unsigned long end_pfn)
>  {
> @@ -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);
> +		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 ",
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs