Re: [PATCH 3/9] mm: alloc_contig_range() added

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

On Thu, 06 Oct 2011 15:54:43 +0200
Marek Szyprowski <m.szyprowski@samsung.com> wrote:

> From: Michal Nazarewicz <m.nazarewicz@samsung.com>
> 
> This commit adds the alloc_contig_range() function which tries
> to allocate given range of pages.  It tries to migrate all
> already allocated pages that fall in the range thus freeing them.
> Once all pages in the range are freed they are removed from the
> buddy system thus allocated for the caller to use.
> 
> Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> [m.szyprowski: renamed some variables for easier code reading]
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> CC: Michal Nazarewicz <mina86@mina86.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>

Where-is: Mel Gorman <mel@csn.ul.ie>

> +#define MIGRATION_RETRY	5
> +static int __alloc_contig_migrate_range(unsigned long start, unsigned long end)
> +{
> +	int migration_failed = 0, ret;
> +	unsigned long pfn = start;
> +
> +	/*
> +	 * Some code "borrowed" from KAMEZAWA Hiroyuki's
> +	 * __alloc_contig_pages().
> +	 */
> +
> +	/* drop all pages in pagevec and pcp list */
> +	lru_add_drain_all();
> +	drain_all_pages();

These operations are sometimes wrong ;) Have you confirmed that we
really need to perform them here?  If so, a little comment explaining
why we're using them here would be good.

> +	for (;;) {
> +		pfn = scan_lru_pages(pfn, end);
> +		if (!pfn || pfn >= end)
> +			break;
> +
> +		ret = do_migrate_range(pfn, end);
> +		if (!ret) {
> +			migration_failed = 0;
> +		} else if (ret != -EBUSY
> +			|| ++migration_failed >= MIGRATION_RETRY) {

Sigh, magic numbers.

Have you ever seen this retry loop actually expire in testing?

migrate_pages() tries ten times.  This code tries five times.  Is there
any science to all of this?

> +			return ret;
> +		} else {
> +			/* There are unstable pages.on pagevec. */
> +			lru_add_drain_all();
> +			/*
> +			 * there may be pages on pcplist before
> +			 * we mark the range as ISOLATED.
> +			 */
> +			drain_all_pages();
> +		}
> +		cond_resched();
> +	}
> +
> +	if (!migration_failed) {
> +		/* drop all pages in pagevec and pcp list */
> +		lru_add_drain_all();
> +		drain_all_pages();

hm.

> +	}
> +
> +	/* Make sure all pages are isolated */
> +	if (WARN_ON(test_pages_isolated(start, end)))
> +		return -EBUSY;
> +
> +	return 0;
> +}
> +
> +/**
> + * alloc_contig_range() -- tries to allocate given range of pages
> + * @start:	start PFN to allocate
> + * @end:	one-past-the-last PFN to allocate
> + * @flags:	flags passed to alloc_contig_freed_pages().
> + *
> + * The PFN range does not have to be pageblock or MAX_ORDER_NR_PAGES
> + * aligned, hovewer it's callers responsibility to guarantee that we

"however"

"however it is the caller's responsibility.."

> + * are the only thread that changes migrate type of pageblocks the
> + * pages fall in.
> + *
> + * Returns zero on success or negative error code.  On success all
> + * pages which PFN is in (start, end) are allocated for the caller and
> + * need to be freed with free_contig_pages().
> + */
>
> ...
>

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