Btrfs: Avoid memcpy where possible in extent_buffers

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -483,15 +483,50 @@ static inline void btrfs_set_##name(struct extent_buffer *eb,		\
 #define BTRFS_SETGET_HEADER_FUNCS(name, type, member, bits)		\
 static inline u##bits btrfs_##name(struct extent_buffer *eb)		\
 {									\
-	__le##bits res;							\
-	read_eb_member(eb, NULL, type, member, &res);			\
-	return le##bits##_to_cpu(res);					\
+	int err;							\
+	char *map_token;						\
+	char *kaddr;							\
+	unsigned long map_start;					\
+	unsigned long map_len;						\
+	unsigned long offset = offsetof(type, member);			\
+	err = map_extent_buffer(eb, offset,				\
+			        sizeof(((type *)0)->member),		\
+				&map_token, &kaddr,			\
+				&map_start, &map_len, KM_USER0);	\
+	if (!err) {							\
+		__le##bits *tmp = (__le##bits *)(kaddr + offset -	\
+					       map_start);		\
+		u##bits res = le##bits##_to_cpu(*tmp);			\
+		unmap_extent_buffer(eb, map_token, KM_USER0);		\
+		return res;						\
+	} else {							\
+		__le##bits res;						\
+		read_eb_member(eb, NULL, type, member, &res);		\
+		return le##bits##_to_cpu(res);				\
+	}								\
 }									\
 static inline void btrfs_set_##name(struct extent_buffer *eb,		\
 				    u##bits val)			\
 {									\
-	val = cpu_to_le##bits(val);					\
-	write_eb_member(eb, NULL, type, member, &val);			\
+	int err;							\
+	char *map_token;						\
+	char *kaddr;							\
+	unsigned long map_start;					\
+	unsigned long map_len;						\
+	unsigned long offset = offsetof(type, member);			\
+	err = map_extent_buffer(eb, offset,				\
+			        sizeof(((type *)0)->member),		\
+				&map_token, &kaddr,			\
+				&map_start, &map_len, KM_USER0);	\
+	if (!err) {							\
+		__le##bits *tmp = (__le##bits *)(kaddr + offset -	\
+					       map_start);		\
+		*tmp = cpu_to_le##bits(val);				\
+		unmap_extent_buffer(eb, map_token, KM_USER0);		\
+	} else {							\
+		val = cpu_to_le##bits(val);				\
+		write_eb_member(eb, NULL, type, member, &val);		\
+	}								\
 }
 
 #define BTRFS_SETGET_STACK_FUNCS(name, type, member, bits)		\

fs/btrfs/disk-io.c

@@ -609,20 +609,20 @@ int close_ctree(struct btrfs_root *root)
 
 int btrfs_buffer_uptodate(struct extent_buffer *buf)
 {
-	struct inode *btree_inode = buf->pages[0]->mapping->host;
+	struct inode *btree_inode = buf->first_page->mapping->host;
 	return extent_buffer_uptodate(&BTRFS_I(btree_inode)->extent_tree, buf);
 }
 
 int btrfs_set_buffer_uptodate(struct extent_buffer *buf)
 {
-	struct inode *btree_inode = buf->pages[0]->mapping->host;
+	struct inode *btree_inode = buf->first_page->mapping->host;
 	return set_extent_buffer_uptodate(&BTRFS_I(btree_inode)->extent_tree,
 					  buf);
 }
 
 void btrfs_mark_buffer_dirty(struct extent_buffer *buf)
 {
-	struct btrfs_root *root = BTRFS_I(buf->pages[0]->mapping->host)->root;
+	struct btrfs_root *root = BTRFS_I(buf->first_page->mapping->host)->root;
 	u64 transid = btrfs_header_generation(buf);
 	struct inode *btree_inode = root->fs_info->btree_inode;
 

fs/btrfs/extent_map.h

@@ -62,14 +62,13 @@ struct extent_state {
 	struct list_head list;
 };
 
-#define EXTENT_BUFFER_SIZE(nr) (sizeof(struct extent_buffer) + \
-			       (nr - 1) * sizeof(struct page *))
 struct extent_buffer {
 	u64 start;
 	unsigned long len;
 	atomic_t refs;
 	int flags;
-	struct page *pages[];
+	struct list_head list;
+	struct page *first_page;
 };
 
 typedef struct extent_map *(get_extent_t)(struct inode *inode,