mm: use alloc_pages_exact() in alloc_large_system_hash() to avoid duplicated logic

alloc_large_system_hash() has logic for freeing pages at the end of an
excessively large power-of-two buffer that is a duplicate of what is in
alloc_pages_exact().  This patch converts alloc_large_system_hash() to use
alloc_pages_exact().

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -4699,26 +4699,13 @@ void *__init alloc_large_system_hash(const char *tablename,
 		else if (hashdist)
 			table = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
 		else {
-			unsigned long order = get_order(size);
-
-			if (order < MAX_ORDER)
-				table = (void *)__get_free_pages(GFP_ATOMIC,
-								order);
 			/*
 			 * If bucketsize is not a power-of-two, we may free
-			 * some pages at the end of hash table.
+			 * some pages at the end of hash table which
+			 * alloc_pages_exact() automatically does
 			 */
-			if (table) {
-				unsigned long alloc_end = (unsigned long)table +
-						(PAGE_SIZE << order);
-				unsigned long used = (unsigned long)table +
-						PAGE_ALIGN(size);
-				split_page(virt_to_page(table), order);
-				while (used < alloc_end) {
-					free_page(used);
-					used += PAGE_SIZE;
-				}
-			}
+			if (get_order(size) < MAX_ORDER)
+				table = alloc_pages_exact(size, GFP_ATOMIC);
 		}
 	} while (!table && size > PAGE_SIZE && --log2qty);