Re: [RFC PATCH 08/26] mm: page_alloc: claim blocks during compaction capturing

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Apr 18, 2023 at 03:12:55PM -0400, Johannes Weiner wrote:
> When capturing a whole block, update the migratetype accordingly. For
> example, a THP allocation might capture an unmovable block. If the THP
> gets split and partially freed later, the remainder should group up
> with movable allocations.
> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/internal.h   |  1 +
>  mm/page_alloc.c | 42 ++++++++++++++++++++++++------------------
>  2 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 024affd4e4b5..39f65a463631 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -432,6 +432,7 @@ struct compact_control {
>   */
>  struct capture_control {
>  	struct compact_control *cc;
> +	int migratetype;
>  	struct page *page;
>  };
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4d20513c83be..8e5996f8b4b4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -615,6 +615,17 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
>  				page_to_pfn(page), MIGRATETYPE_MASK);
>  }
>  
> +static void change_pageblock_range(struct page *pageblock_page,
> +					int start_order, int migratetype)
> +{
> +	int nr_pageblocks = 1 << (start_order - pageblock_order);
> +
> +	while (nr_pageblocks--) {
> +		set_pageblock_migratetype(pageblock_page, migratetype);
> +		pageblock_page += pageblock_nr_pages;
> +	}
> +}
> +
>  #ifdef CONFIG_DEBUG_VM
>  static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
>  {
> @@ -962,14 +973,19 @@ compaction_capture(struct capture_control *capc, struct page *page,
>  	    is_migrate_isolate(migratetype))
>  		return false;
>  
> -	/*
> -	 * Do not let lower order allocations pollute a movable pageblock.
> -	 * This might let an unmovable request use a reclaimable pageblock
> -	 * and vice-versa but no more than normal fallback logic which can
> -	 * have trouble finding a high-order free page.
> -	 */
> -	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE)
> +	if (order >= pageblock_order) {
> +		migratetype = capc->migratetype;
> +		change_pageblock_range(page, order, migratetype);
> +	} else if (migratetype == MIGRATE_MOVABLE) {
> +		/*
> +		 * Do not let lower order allocations pollute a
> +		 * movable pageblock.  This might let an unmovable
> +		 * request use a reclaimable pageblock and vice-versa
> +		 * but no more than normal fallback logic which can
> +		 * have trouble finding a high-order free page.
> +		 */
>  		return false;
> +	}
>  

For capturing pageblock order or larger, why not unconditionally make
the block MOVABLE? Even if it's a zero page allocation, it would be nice
to keep the pageblock for movable pages after the split as long as
possible.

-- 
Mel Gorman
SUSE Labs