[PATCH] Sanely size hash tables when using large base pages

At the moment the inode/dentry cache hash tables (common by way of
alloc_large_system_hash()) are incorrectly sized by their respective
detection logic when we attempt to use large base pages on systems with
little memory.

This results in odd behaviour when using a 64kB PAGE_SIZE, such as:

Dentry cache hash table entries: 8192 (order: -1, 32768 bytes)
Inode-cache hash table entries: 4096 (order: -2, 16384 bytes)

The mount cache hash table is seemingly the only one that gets this right
by directly taking PAGE_SIZE in to account.

The following patch attempts to catch the bogus values and round it up to
at least 0-order.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/page_alloc.c

@@ -3321,6 +3321,10 @@ void *__init alloc_large_system_hash(const char *tablename,
 			numentries >>= (scale - PAGE_SHIFT);
 		else
 			numentries <<= (PAGE_SHIFT - scale);
+
+		/* Make sure we've got at least a 0-order allocation.. */
+		if (unlikely((numentries * bucketsize) < PAGE_SIZE))
+			numentries = PAGE_SIZE / bucketsize;
 	}
 	numentries = roundup_pow_of_two(numentries);