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

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Date:	Mon, 05 May 2014 16:20:04 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	linux-kernel@...r.kernel.org
CC:	iamjoonsoo.kim@....com, sparclinux@...r.kernel.org,
	hannes@...xchg.org, cl@...ux.com, penberg@...nel.org,
	torvalds@...ux-foundation.org
Subject: [PATCH] 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.

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