ext4: rework map struct instantiation in ext4_ext_map_blocks()

The path performing block allocations in ext4_ext_map_blocks() contains
code trimming the length of a new extent that is repeated later
in the function.  This code is both redundant and unnecessary as the
exact length of the new extent has already been calculated.  Rewrite the
instantiation of the map struct in this case to use the available
values, avoiding the overhead of unnecessary conversions and improving
clarity.  Add another map struct instantiation tailored specifically to
the separate case for an existing written extent.  Remove an old comment
that no longer appears applicable to the current code.
Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Link: https://lore.kernel.org/r/20200510155805.18808-1-enwlinux@gmail.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>

fs/ext4/extents.c

@@ -4045,7 +4045,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	struct ext4_ext_path *path = NULL;
 	struct ext4_extent newex, *ex, *ex2;
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
-	ext4_fsblk_t newblock = 0;
+	ext4_fsblk_t newblock = 0, pblk;
 	int err = 0, depth, ret;
 	unsigned int allocated = 0, offset = 0;
 	unsigned int allocated_clusters = 0;
@@ -4060,7 +4060,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	if (IS_ERR(path)) {
 		err = PTR_ERR(path);
 		path = NULL;
-		goto out2;
+		goto out;
 	}
 
 	depth = ext_depth(inode);
@@ -4076,7 +4076,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 				 (unsigned long) map->m_lblk, depth,
 				 path[depth].p_block);
 		err = -EFSCORRUPTED;
-		goto out2;
+		goto out;
 	}
 
 	ex = path[depth].p_ext;
@@ -4110,8 +4110,14 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 			    (flags & EXT4_GET_BLOCKS_CONVERT_UNWRITTEN)) {
 				err = convert_initialized_extent(handle,
 					inode, map, &path, &allocated);
-				goto out2;
+				goto out;
 			} else if (!ext4_ext_is_unwritten(ex)) {
+				map->m_flags |= EXT4_MAP_MAPPED;
+				map->m_pblk = newblock;
+				if (allocated > map->m_len)
+					allocated = map->m_len;
+				map->m_len = allocated;
+				ext4_ext_show_leaf(inode, path);
 				goto out;
 			}
 
@@ -4122,7 +4128,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 				err = ret;
 			else
 				allocated = ret;
-			goto out2;
+			goto out;
 		}
 	}
 
@@ -4147,7 +4153,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		map->m_pblk = 0;
 		map->m_len = min_t(unsigned int, map->m_len, hole_len);
 
-		goto out2;
+		goto out;
 	}
 
 	/*
@@ -4171,12 +4177,12 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	ar.lleft = map->m_lblk;
 	err = ext4_ext_search_left(inode, path, &ar.lleft, &ar.pleft);
 	if (err)
-		goto out2;
+		goto out;
 	ar.lright = map->m_lblk;
 	ex2 = NULL;
 	err = ext4_ext_search_right(inode, path, &ar.lright, &ar.pright, &ex2);
 	if (err)
-		goto out2;
+		goto out;
 
 	/* Check if the extent after searching to the right implies a
 	 * cluster we can use. */
@@ -4237,7 +4243,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		ar.flags |= EXT4_MB_USE_RESERVED;
 	newblock = ext4_mb_new_blocks(handle, &ar, &err);
 	if (!newblock)
-		goto out2;
+		goto out;
 	allocated_clusters = ar.len;
 	ar.len = EXT4_C2B(sbi, ar.len) - offset;
 	ext_debug(inode, "allocate new block: goal %llu, found %llu/%u, requested %u\n",
@@ -4247,7 +4253,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 
 got_allocated_blocks:
 	/* try to insert new extent into found leaf and return */
-	ext4_ext_store_pblock(&newex, newblock + offset);
+	pblk = newblock + offset;
+	ext4_ext_store_pblock(&newex, pblk);
 	newex.ee_len = cpu_to_le16(ar.len);
 	/* Mark unwritten */
 	if (flags & EXT4_GET_BLOCKS_UNWRIT_EXT) {
@@ -4272,16 +4279,9 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 					 EXT4_C2B(sbi, allocated_clusters),
 					 fb_flags);
 		}
-		goto out2;
+		goto out;
 	}
 
-	/* previous routine could use block we allocated */
-	newblock = ext4_ext_pblock(&newex);
-	allocated = ext4_ext_get_actual_len(&newex);
-	if (allocated > map->m_len)
-		allocated = map->m_len;
-	map->m_flags |= EXT4_MAP_NEW;
-
 	/*
 	 * Reduce the reserved cluster count to reflect successful deferred
 	 * allocation of delayed allocated clusters or direct allocation of
@@ -4327,14 +4327,14 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 		ext4_update_inode_fsync_trans(handle, inode, 1);
 	else
 		ext4_update_inode_fsync_trans(handle, inode, 0);
-out:
-	if (allocated > map->m_len)
-		allocated = map->m_len;
+
+	map->m_flags |= (EXT4_MAP_NEW | EXT4_MAP_MAPPED);
+	map->m_pblk = pblk;
+	map->m_len = ar.len;
+	allocated = map->m_len;
 	ext4_ext_show_leaf(inode, path);
-	map->m_flags |= EXT4_MAP_MAPPED;
-	map->m_pblk = newblock;
-	map->m_len = allocated;
-out2:
+
+out:
 	ext4_ext_drop_refs(path);
 	kfree(path);