Sanely size hash tables when using large base pages. take 2.

This is a resubmission of the earlier patch with the pidhash bits dropped.

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.

--

 mm/page_alloc.c |    4 ++++
 1 file changed, 4 insertions(+)

index 8c1a116..4a9a83f 100644
@@ -3321,6 +3321,10 @@ void *__init alloc_large_system_hash(con
 			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);
 

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