Re: [PATCH 4/9] mm: MIGRATE_CMA migration type added

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 06 Oct 2011 15:54:44 +0200
Marek Szyprowski <m.szyprowski@samsung.com> wrote:

> From: Michal Nazarewicz <m.nazarewicz@samsung.com>
> 
> The MIGRATE_CMA migration type has two main characteristics:
> (i) only movable pages can be allocated from MIGRATE_CMA
> pageblocks and (ii) page allocator will never change migration
> type of MIGRATE_CMA pageblocks.
> 
> This guarantees that page in a MIGRATE_CMA page block can
> always be migrated somewhere else (unless there's no memory left
> in the system).
> 
> It is designed to be used with Contiguous Memory Allocator
> (CMA) for allocating big chunks (eg. 10MiB) of physically
> contiguous memory.  Once driver requests contiguous memory,
> CMA will migrate pages from MIGRATE_CMA pageblocks.
> 
> To minimise number of migrations, MIGRATE_CMA migration type
> is the last type tried when page allocator falls back to other
> migration types then requested.
> 
>
> ...
>
> +#ifdef CONFIG_CMA_MIGRATE_TYPE
> +#  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
> +#else
> +#  define is_migrate_cma(migratetype) false
> +#endif

Implement in C, please.

>
> ...
>
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -115,6 +115,16 @@ static bool suitable_migration_target(struct page *page)
>  	if (migratetype == MIGRATE_ISOLATE || migratetype == MIGRATE_RESERVE)
>  		return false;
>  
> +	/* Keep MIGRATE_CMA alone as well. */
> +	/*
> +	 * XXX Revisit.  We currently cannot let compaction touch CMA
> +	 * pages since compaction insists on changing their migration
> +	 * type to MIGRATE_MOVABLE (see split_free_page() called from
> +	 * isolate_freepages_block() above).
> +	 */

Talk to us about this.

How serious is this shortcoming in practice?  What would a fix look
like?  Is anyone working on an implementation, or planning to do so?


> +	if (is_migrate_cma(migratetype))
> +		return false;
> +
>  	/* If the page is a large free page, then allow migration */
>  	if (PageBuddy(page) && page_order(page) >= pageblock_order)
>  		return true;
>
> ...
>
> +void __init init_cma_reserved_pageblock(struct page *page)
> +{
> +	struct page *p = page;
> +	unsigned i = pageblock_nr_pages;
> +
> +	prefetchw(p);
> +	do {
> +		if (--i)
> +			prefetchw(p + 1);
> +		__ClearPageReserved(p);
> +		set_page_count(p, 0);
> +	} while (++p, i);
> +
> +	set_page_refcounted(page);
> +	set_pageblock_migratetype(page, MIGRATE_CMA);
> +	__free_pages(page, pageblock_order);
> +	totalram_pages += pageblock_nr_pages;
> +}

I wonder if the prefetches do any good.  it doesn't seem very important
in an __init function.

> +#endif
>  
>
> ...
>

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