Re: [PATCH 4/8] mm: page_alloc: fix up block types when merging compatible blocks

[Zi Yan <ziy@nvidia.com>]

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On 21 Aug 2023, at 14:33, Johannes Weiner wrote:

> The buddy allocator coalesces compatible blocks during freeing, but it
> doesn't update the types of the subblocks to match. When an allocation
> later breaks the chunk down again, its pieces will be put on freelists
> of the wrong type. This encourages incompatible page mixing (ask for
> one type, get another), and thus long-term fragmentation.
>
> Update the subblocks when merging a larger chunk, such that a later
> expand() will maintain freelist type hygiene.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  mm/page_alloc.c | 37 ++++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a5e36d186893..6c9f565b2613 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -438,6 +438,17 @@ 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;
> +	}
> +}
> +

Is this code move included by accident?

>  #ifdef CONFIG_DEBUG_VM
>  static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
>  {
> @@ -808,10 +819,17 @@ static inline void __free_one_page(struct page *page,
>  			 */
>  			int buddy_mt = get_pfnblock_migratetype(buddy, buddy_pfn);
>
> -			if (migratetype != buddy_mt
> -					&& (!migratetype_is_mergeable(migratetype) ||
> -						!migratetype_is_mergeable(buddy_mt)))
> -				goto done_merging;
> +			if (migratetype != buddy_mt) {
> +				if (!migratetype_is_mergeable(migratetype) ||
> +				    !migratetype_is_mergeable(buddy_mt))
> +					goto done_merging;
> +				/*
> +				 * Match buddy type. This ensures that
> +				 * an expand() down the line puts the
> +				 * sub-blocks on the right freelists.
> +				 */
> +				set_pageblock_migratetype(buddy, migratetype);
> +			}
>  		}
>
>  		/*
> @@ -1687,17 +1705,6 @@ int move_freepages_block(struct zone *zone, struct page *page,
>  								num_movable);
>  }
>
> -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;
> -	}
> -}
> -
>  /*
>   * When we are falling back to another migratetype during allocation, try to
>   * steal extra free pages from the same pageblocks to satisfy further
> -- 
> 2.41.0


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Best Regards,
Yan, Zi

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