linux-kernel - [PATCH 1/3] shmem: fix division by zero

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Date:	Mon, 6 Apr 2009 21:54:54 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Yuri Tikhonov <yur@...raft.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] shmem: fix division by zero

From: Yuri Tikhonov <yur@...raft.com>

Fix a division by zero which we have in shmem_truncate_range() and
shmem_unuse_inode() when using big PAGE_SIZE values (e.g. 256kB on ppc44x).

With 256kB PAGE_SIZE, the ENTRIES_PER_PAGEPAGE constant becomes too large
(0x1.0000.0000) on a 32-bit kernel, so this patch just changes its type
from 'unsigned long' to 'unsigned long long'.

Hugh: reverted its unsigned long longs in shmem_truncate_range() and
shmem_getpage(): the pagecache index cannot be more than an unsigned long,
so the divisions by zero occurred in unreached code.  It's a pity we need
any ULL arithmetic here, but I found no pretty way to avoid it.

Signed-off-by: Yuri Tikhonov <yur@...raft.com>
Signed-off-by: Hugh Dickins <hugh@...itas.com>
---

 mm/shmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 2.6.29-git13/mm/shmem.c	2009-04-06 11:48:55.000000000 +0100
+++ 2.6.29-git13+/mm/shmem.c	2009-04-06 15:33:09.000000000 +0100
@@ -66,7 +66,7 @@ static struct vfsmount *shm_mnt;
 #include <asm/pgtable.h>

 #define ENTRIES_PER_PAGE (PAGE_CACHE_SIZE/sizeof(unsigned long))
-#define ENTRIES_PER_PAGEPAGE (ENTRIES_PER_PAGE*ENTRIES_PER_PAGE)
+#define ENTRIES_PER_PAGEPAGE ((unsigned long long)ENTRIES_PER_PAGE*ENTRIES_PER_PAGE)
 #define BLOCKS_PER_PAGE  (PAGE_CACHE_SIZE/512)

 #define SHMEM_MAX_INDEX  (SHMEM_NR_DIRECT + (ENTRIES_PER_PAGEPAGE/2) * (ENTRIES_PER_PAGE+1))
--
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