linux-kernel - Re: [PATCH] mm/page_alloc: Do not isolate redundant pageblock

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Date:   Fri, 27 Nov 2020 17:54:45 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: Do not isolate redundant pageblock

On 27.11.20 15:19, Muchun Song wrote:
> Current pageblock isolation logic could isolate each pageblock individually
> since commit d9dddbf55667 ("mm/page_alloc: prevent merging between isolated
> and other pageblocks"). So we not need to concern about page allocator
> merges buddies from different pageblocks and changes MIGRATE_ISOLATE to
> some other migration type.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/page_alloc.c | 26 ++++++++------------------
>  1 file changed, 8 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cefbef32bf4a..608a2c2b8ab7 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8313,16 +8313,14 @@ struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>  }
>  
>  #ifdef CONFIG_CONTIG_ALLOC
> -static unsigned long pfn_max_align_down(unsigned long pfn)
> +static unsigned long pfn_align_down(unsigned long pfn)
>  {
> -	return pfn & ~(max_t(unsigned long, MAX_ORDER_NR_PAGES,
> -			     pageblock_nr_pages) - 1);
> +	return pfn & ~(pageblock_nr_pages - 1);
>  }
>  
> -static unsigned long pfn_max_align_up(unsigned long pfn)
> +static unsigned long pfn_align_up(unsigned long pfn)
>  {
> -	return ALIGN(pfn, max_t(unsigned long, MAX_ORDER_NR_PAGES,
> -				pageblock_nr_pages));
> +	return ALIGN(pfn, pageblock_nr_pages);
>  }
>  
>  /* [start, end) must belong to a single zone. */
> @@ -8415,14 +8413,6 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	INIT_LIST_HEAD(&cc.migratepages);
>  
>  	/*
> -	 * What we do here is we mark all pageblocks in range as
> -	 * MIGRATE_ISOLATE.  Because pageblock and max order pages may
> -	 * have different sizes, and due to the way page allocator
> -	 * work, we align the range to biggest of the two pages so
> -	 * that page allocator won't try to merge buddies from
> -	 * different pageblocks and change MIGRATE_ISOLATE to some
> -	 * other migration type.
> -	 *
>  	 * Once the pageblocks are marked as MIGRATE_ISOLATE, we
>  	 * migrate the pages from an unaligned range (ie. pages that
>  	 * we are interested in).  This will put all the pages in
> @@ -8438,8 +8428,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  	 * put back to page allocator so that buddy can use them.
>  	 */
>  
> -	ret = start_isolate_page_range(pfn_max_align_down(start),
> -				       pfn_max_align_up(end), migratetype, 0);
> +	ret = start_isolate_page_range(pfn_align_down(start), pfn_align_up(end),
> +				       migratetype, 0);
>  	if (ret)
>  		return ret;
>  
> @@ -8522,8 +8512,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>  		free_contig_range(end, outer_end - end);
>  
>  done:
> -	undo_isolate_page_range(pfn_max_align_down(start),
> -				pfn_max_align_up(end), migratetype);
> +	undo_isolate_page_range(pfn_align_down(start), pfn_align_up(end),
> +				migratetype);
>  	return ret;
>  }
>  EXPORT_SYMBOL(alloc_contig_range);
> 

Last time I checked, set_migratetype_isolate()->has_unmovable_pages()
was not prepared for that in case of !CMA and !ZONE_MOVABLE.

Assume you have an unmovable MAX_ORDER - 1 page that spans two
pageblocks (e.g., x86-64). Assume you try to isolate the second
pageblock. IIRC, you would answer "yes", as the refcount of all involved
pages is 0.


How did you test this works as expected?

-- 
Thanks,

David / dhildenb