[PATCH]Fix for map_private_extent_buffer to handle extent which is not page alignment

["Liu Hui" <onlyflyer@gmail.com>]

Hi,
Now, BTRFS only supports page alignment extent. But in
map_private_extent(), there are already some codes to handle the
extent which is not page alignment. For example,
map_private_extent_buffer also wants to handle a extent which is 7K
length properly. The problem is the codes are not enough, e.g. in the
following figure, map_private_extent_buffer can handle extent1, but it
can't take the right behavior to handle extent2. This patch fixs it.

PS:Please use the mono font to view the ASCII figures, thanks.

   |-extent1-|
+-----++-----+
|page1||page2|
+-----++-----+

|-extent2-|
+-----++-----+
|page1||page2|
+-----++-----+

-- 
Thanks & Best Regards
Liu Hui
--

diff --git a/extent_io.c b/extent_io.c
index c3dfe2a..cac47d1 100644
--- a/extent_io.c
+++ b/extent_io.c
@@ -3426,9 +3426,11 @@ int map_private_extent_buffer(struct
extent_buffer *eb, unsigned long start,
 	char *kaddr;
 	struct page *p;
 	size_t start_offset = eb->start & ((u64)PAGE_CACHE_SIZE - 1);
+	size_t last_offset;
 	unsigned long i = (start_offset + start) >> PAGE_CACHE_SHIFT;
 	unsigned long end_i = (start_offset + start + min_len - 1) >>
 		PAGE_CACHE_SHIFT;
+	unsigned long last_i = (start_offset + eb->len - 1) >> PAGE_CACHE_SHIFT;

 	if (i != end_i)
 		return -EINVAL;
@@ -3440,8 +3442,16 @@ int map_private_extent_buffer(struct
extent_buffer *eb, unsigned long start,
 		offset = 0;
 		*map_start = ((u64)i << PAGE_CACHE_SHIFT) - start_offset;
 	}
+
+	if (i == last_i)
+		last_offset = ((u64)(i + 1) << PAGE_CACHE_SHIFT) - start_offset
+			      - eb->len;
+	else
+		last_offset = 0;
+
 	if (start + min_len > eb->len) {
-printk("bad mapping eb start %Lu len %lu, wanted %lu %lu\n",
eb->start, eb->len, start, min_len);
+		printk("bad mapping eb start %Lu len %lu, wanted %lu %lu\n",
+		       eb->start, eb->len, start, min_len);
 		WARN_ON(1);
 	}

@@ -3449,7 +3459,7 @@ printk("bad mapping eb start %Lu len %lu, wanted
%lu %lu\n", eb->start, eb->len,
 	kaddr = kmap_atomic(p, km);
 	*token = kaddr;
 	*map = kaddr + offset;
-	*map_len = PAGE_CACHE_SIZE - offset;
+	*map_len = PAGE_CACHE_SIZE - offset - last_offset;
 	return 0;
 }
 EXPORT_SYMBOL(map_private_extent_buffer);