MM: alloc_large_system_hash() can free some memory for non power-of-two bucketsize

alloc_large_system_hash() is called at boot time to allocate space for
several large hash tables.

Lately, TCP hash table was changed and its bucketsize is not a power-of-two
anymore.

On most setups, alloc_large_system_hash() allocates one big page (order >
0) with __get_free_pages(GFP_ATOMIC, order).  This single high_order page
has a power-of-two size, bigger than the needed size.

We can free all pages that wont be used by the hash table.

On a 1GB i386 machine, this patch saves 128 KB of LOWMEM memory.

TCP established hash table entries: 32768 (order: 6, 393216 bytes)
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3584,6 +3584,21 @@ void *__init alloc_large_system_hash(const char *tablename,
 			for (order = 0; ((1UL << order) << PAGE_SHIFT) < size; 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.
+			 */
+			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;
+				}
+			}
 		}
 	} while (!table && size > PAGE_SIZE && --log2qty);