Btrfs: [PATCH] extent_map: provide generic bmap

generic_bmap is completely trivial, while the extent to bh mapping in
btrfs is rather complex.  So provide a extent_bmap instead that takes
a get_extent callback and can be used by filesystem using the extent_map
code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_map.c

@@ -1816,3 +1816,22 @@ int try_release_extent_mapping(struct extent_map_tree *tree, struct page *page)
 }
 EXPORT_SYMBOL(try_release_extent_mapping);
 
+sector_t extent_bmap(struct address_space *mapping, sector_t iblock,
+		get_extent_t *get_extent)
+{
+	struct inode *inode = mapping->host;
+	u64 start = iblock << inode->i_blkbits;
+	u64 end = start + (1 << inode->i_blkbits) - 1;
+	struct extent_map *em;
+
+	em = get_extent(inode, NULL, 0, start, end, 0);
+	if (!em || IS_ERR(em))
+		return 0;
+
+	// XXX(hch): block 0 is valid in some cases, e.g. XFS RT device
+	if (em->block_start == EXTENT_MAP_INLINE ||
+	    em->block_start == 0)
+	    	return 0;
+
+	return (em->block_start + start - em->start) >> inode->i_blkbits;
+}

fs/btrfs/extent_map.h

@@ -99,6 +99,8 @@ int extent_prepare_write(struct extent_map_tree *tree,
 int extent_commit_write(struct extent_map_tree *tree,
 			struct inode *inode, struct page *page,
 			unsigned from, unsigned to);
+sector_t extent_bmap(struct address_space *mapping, sector_t iblock,
+		get_extent_t *get_extent);
 int set_range_dirty(struct extent_map_tree *tree, u64 start, u64 end);
 int set_state_private(struct extent_map_tree *tree, u64 start, u64 private);
 int get_state_private(struct extent_map_tree *tree, u64 start, u64 *private);

fs/btrfs/inode.c

@@ -1567,28 +1567,9 @@ out:
 	return em;
 }
 
-static int btrfs_get_block_bmap(struct inode *inode, sector_t iblock,
-			   struct buffer_head *result, int create)
+static sector_t btrfs_bmap(struct address_space *mapping, sector_t iblock)
 {
-	struct btrfs_root *root = BTRFS_I(inode)->root;
-	u64 start = iblock << inode->i_blkbits;
-	u64 end = start + root->blocksize -1;
-	struct extent_map *em;
-
-	em = btrfs_get_extent(inode, NULL, 0, start, end, 0);
-	if (em && !IS_ERR(em) && em->block_start != EXTENT_MAP_INLINE &&
-	    em->block_start != 0) {
-		u64 offset;
-		offset = start - em->start;
-		start = (em->block_start + offset) >> inode->i_blkbits;
-		btrfs_map_bh_to_logical(root, result, start);
-	}
-	return 0;
-}
-
-static sector_t btrfs_bmap(struct address_space *as, sector_t block)
-{
-	return generic_block_bmap(as, block, btrfs_get_block_bmap);
+	return extent_bmap(mapping, iblock, btrfs_get_extent);
 }
 
 static int btrfs_prepare_write(struct file *file, struct page *page,