slab: Fix off by one in object max number tests.

If freelist_idx_t is a byte, SLAB_OBJ_MAX_NUM should be 255 not 256, and
likewise if freelist_idx_t is a short, then it should be 65535 not
65536.

This was leading to all kinds of random crashes on sparc64 where
PAGE_SIZE is 8192.  One problem shown was that if spinlock debugging was
enabled, we'd get deadlocks in copy_pte_range() or do_wp_page() with the
same cpu already holding a lock it shouldn't hold, or the lock belonging
to a completely unrelated process.

Fixes: a41adfa ("slab: introduce byte sized index for the freelist of a slab")
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/slab.c

@@ -166,7 +166,7 @@ typedef unsigned char freelist_idx_t;
 typedef unsigned short freelist_idx_t;
 #endif
 
-#define SLAB_OBJ_MAX_NUM (1 << sizeof(freelist_idx_t) * BITS_PER_BYTE)
+#define SLAB_OBJ_MAX_NUM ((1 << sizeof(freelist_idx_t) * BITS_PER_BYTE) - 1)
 
 /*
  * true if a page was allocated from pfmemalloc reserves for network-based