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Subject: Re: [PATCH RFC 3/4] mm/page_alloc: always move pages to the tail of
 the freelist in unset_migratetype_isolate()
To:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, linux-hyperv@vger.kernel.org,
        xen-devel@lists.xenproject.org, linux-acpi@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Wei Yang <richard.weiyang@linux.alibaba.com>,
        Oscar Salvador <osalvador@suse.de>,
        Mike Rapoport <rppt@kernel.org>,
        Scott Cheloha <cheloha@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>
References: <20200916183411.64756-1-david@redhat.com>
 <20200916183411.64756-4-david@redhat.com>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <9c6cc094-b02a-ac6c-e1ca-370ce7257881@suse.cz>
Date:   Thu, 24 Sep 2020 13:13:29 +0200
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On 9/16/20 8:34 PM, David Hildenbrand wrote:
> Page isolation doesn't actually touch the pages, it simply isolates
> pageblocks and moves all free pages to the MIGRATE_ISOLATE freelist.
> 
> We already place pages to the tail of the freelists when undoing
> isolation via __putback_isolated_page(), let's do it in any case
> (e.g., if order == pageblock_order) and document the behavior.
> 
> This change results in all pages getting onlined via online_pages() to
> be placed to the tail of the freelist.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Scott Cheloha <cheloha@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/page-isolation.h |  2 ++
>  mm/page_alloc.c                | 36 +++++++++++++++++++++++++++++-----
>  mm/page_isolation.c            |  8 ++++++--
>  3 files changed, 39 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
> index 572458016331..a36be2cf4dbb 100644
> --- a/include/linux/page-isolation.h
> +++ b/include/linux/page-isolation.h
> @@ -38,6 +38,8 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  void set_pageblock_migratetype(struct page *page, int migratetype);
>  int move_freepages_block(struct zone *zone, struct page *page,
>  				int migratetype, int *num_movable);
> +int move_freepages_block_tail(struct zone *zone, struct page *page,
> +			      int migratetype);
>  
>  /*
>   * Changes migrate type in [start_pfn, end_pfn) to be MIGRATE_ISOLATE.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index bba9a0f60c70..75b0f49b4022 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -899,6 +899,15 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
>  	list_move(&page->lru, &area->free_list[migratetype]);
>  }
>  
> +/* Used for pages which are on another list */
> +static inline void move_to_free_list_tail(struct page *page, struct zone *zone,
> +					  unsigned int order, int migratetype)
> +{
> +	struct free_area *area = &zone->free_area[order];
> +
> +	list_move_tail(&page->lru, &area->free_list[migratetype]);
> +}

There are just 3 callers of move_to_free_list() before this patch, I would just
add the to_tail parameter there instead of new wrapper. For callers with
constant parameter, the inline will eliminate it anyway.

>  static inline void del_page_from_free_list(struct page *page, struct zone *zone,
>  					   unsigned int order)
>  {
> @@ -2323,7 +2332,7 @@ static inline struct page *__rmqueue_cma_fallback(struct zone *zone,
>   */
>  static int move_freepages(struct zone *zone,
>  			  struct page *start_page, struct page *end_page,
> -			  int migratetype, int *num_movable)
> +			  int migratetype, int *num_movable, bool to_tail)
>  {
>  	struct page *page;
>  	unsigned int order;
> @@ -2354,7 +2363,10 @@ static int move_freepages(struct zone *zone,
>  		VM_BUG_ON_PAGE(page_zone(page) != zone, page);
>  
>  		order = page_order(page);
> -		move_to_free_list(page, zone, order, migratetype);
> +		if (to_tail)
> +			move_to_free_list_tail(page, zone, order, migratetype);
> +		else
> +			move_to_free_list(page, zone, order, migratetype);
>  		page += 1 << order;
>  		pages_moved += 1 << order;
>  	}
> @@ -2362,8 +2374,9 @@ static int move_freepages(struct zone *zone,
>  	return pages_moved;
>  }
>  
> -int move_freepages_block(struct zone *zone, struct page *page,
> -				int migratetype, int *num_movable)
> +static int __move_freepages_block(struct zone *zone, struct page *page,
> +				  int migratetype, int *num_movable,
> +				  bool to_tail)
>  {
>  	unsigned long start_pfn, end_pfn;
>  	struct page *start_page, *end_page;
> @@ -2384,7 +2397,20 @@ int move_freepages_block(struct zone *zone, struct page *page,
>  		return 0;
>  
>  	return move_freepages(zone, start_page, end_page, migratetype,
> -								num_movable);
> +			      num_movable, to_tail);
> +}
> +
> +int move_freepages_block(struct zone *zone, struct page *page,
> +			 int migratetype, int *num_movable)
> +{
> +	return __move_freepages_block(zone, page, migratetype, num_movable,
> +				      false);
> +}
> +
> +int move_freepages_block_tail(struct zone *zone, struct page *page,
> +			      int migratetype)
> +{
> +	return __move_freepages_block(zone, page, migratetype, NULL, true);
>  }

Likewise, just 5 callers of move_freepages_block(), all in the files you're
already changing, so no need for this wrappers IMHO.

Thanks,
Vlastimil

>  static void change_pageblock_range(struct page *pageblock_page,
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index abfe26ad59fd..84aa1d14751d 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -83,7 +83,7 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	 * Because freepage with more than pageblock_order on isolated
>  	 * pageblock is restricted to merge due to freepage counting problem,
>  	 * it is possible that there is free buddy page.
> -	 * move_freepages_block() doesn't care of merge so we need other
> +	 * move_freepages_block*() don't care about merging, so we need another
>  	 * approach in order to merge them. Isolation and free will make
>  	 * these pages to be merged.
>  	 */
> @@ -106,9 +106,13 @@ static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
>  	 * If we isolate freepage with more than pageblock_order, there
>  	 * should be no freepage in the range, so we could avoid costly
>  	 * pageblock scanning for freepage moving.
> +	 *
> +	 * We didn't actually touch any of the isolated pages, so place them
> +	 * to the tail of the freelists. This is especially relevant during
> +	 * memory onlining.
>  	 */
>  	if (!isolated_page) {
> -		nr_pages = move_freepages_block(zone, page, migratetype, NULL);
> +		nr_pages = move_freepages_block_tail(zone, page, migratetype);
>  		__mod_zone_freepage_state(zone, nr_pages, migratetype);
>  	}
>  	set_pageblock_migratetype(page, migratetype);
>