[RFC:PATCH 008/012] bh_offset needs to take page_address into consideration

[Dave Kleikamp <shaggy@linux.vnet.ibm.com>]

bh_offset needs to take page_address into consideration

ToDo: Check how well gcc optimizes bh_offset when CONFIG_VM_FILE_TAILS is
not defined.  Some optimization may be needed, but we want to avoid
unnecessary ifdefs.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
---

 include/linux/buffer_head.h |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff -Nurp linux007/include/linux/buffer_head.h linux008/include/linux/buffer_head.h
--- linux007/include/linux/buffer_head.h	2007-05-21 15:15:43.000000000 -0500
+++ linux008/include/linux/buffer_head.h	2007-05-23 22:53:12.000000000 -0500
@@ -129,7 +129,17 @@ BUFFER_FNS(Ordered, ordered)
 BUFFER_FNS(Eopnotsupp, eopnotsupp)
 BUFFER_FNS(Unwritten, unwritten)
 
-#define bh_offset(bh)		((unsigned long)(bh)->b_data & ~PAGE_MASK)
+/*
+ * If CONFIG_VM_FILE_TAILS is defined, page_address(bh) may not be
+ * aligned to PAGE_SIZE, so bh_offset must take that into account.
+ */
+static inline unsigned long bh_offset(struct buffer_head *bh)
+{
+	return ((unsigned long)bh->b_data -
+		(unsigned long)page_address(bh->b_page)) &
+	       ~PAGE_MASK;
+}
+
 #define touch_buffer(bh)	mark_page_accessed(bh->b_page)
 
 /* If we *know* page->private refers to buffer_heads */

-- 
David Kleikamp
IBM Linux Technology Center

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