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

David Miller
2014-05-05T20:20:10Z

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.

Fixes: a41adfa ("slab: introduce byte sized index for the freelist of a slab")
Signed-off-by: David S. Miller <davem@davemloft.net>
---

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.

diff --git a/mm/slab.c b/mm/slab.c
index 388cb1a..37de3a7 100644
--- a/mm/slab.c
+++ b/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