On Thu, 06 Oct 2011 15:54:43 +0200
Marek Szyprowski [off-list ref] wrote:

From: Michal Nazarewicz <redacted>

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 <redacted>
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 <redacted>
Acked-by: Arnd Bergmann <arnd@arndb.de>

Where-is: Mel Gorman [off-list ref]

+#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().
+ */

...