ext4: Fix data corruption when writing to prealloc area

Inserting an extent can cause a new entry in the already existing index
block. That doesn't increase the depth of the instead. Instead it adds a
new leaf block. Now with the new leaf block the path information
corresponding to the logical block should be fetched from the new block.
The old path will be pointing to the old leaf block.

We need to recalucate the path information on extent insert
even if depth doesn't change. Without this change, the extent merge
after converting an unwritten extent to initialized extent takes the wrong
extent and cause data corruption.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -2323,7 +2323,10 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 		unsigned int newdepth;
 		/* If extent has less than EXT4_EXT_ZERO_LEN zerout directly */
 		if (allocated <= EXT4_EXT_ZERO_LEN) {
-			/* Mark first half uninitialized.
+			/*
+			 * iblock == ee_block is handled by the zerouout
+			 * at the beginning.
+			 * Mark first half uninitialized.
 			 * Mark second half initialized and zero out the
 			 * initialized extent
 			 */
@@ -2346,7 +2349,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 				ex->ee_len   = orig_ex.ee_len;
 				ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
 				ext4_ext_dirty(handle, inode, path + depth);
-				/* zeroed the full extent */
+				/* blocks available from iblock */
 				return allocated;
 
 			} else if (err)
@@ -2374,6 +2377,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 					err = PTR_ERR(path);
 					return err;
 				}
+				/* get the second half extent details */
 				ex = path[depth].p_ext;
 				err = ext4_ext_get_access(handle, inode,
 								path + depth);
@@ -2403,6 +2407,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 			ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
 			ext4_ext_dirty(handle, inode, path + depth);
 			/* zeroed the full extent */
+			/* blocks available from iblock */
 			return allocated;
 
 		} else if (err)
@@ -2418,23 +2423,22 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 		 */
 		orig_ex.ee_len = cpu_to_le16(ee_len -
 						ext4_ext_get_actual_len(ex3));
-		if (newdepth != depth) {
-			depth = newdepth;
-			ext4_ext_drop_refs(path);
-			path = ext4_ext_find_extent(inode, iblock, path);
-			if (IS_ERR(path)) {
-				err = PTR_ERR(path);
-				goto out;
-			}
-			eh = path[depth].p_hdr;
-			ex = path[depth].p_ext;
-			if (ex2 != &newex)
-				ex2 = ex;
-
-			err = ext4_ext_get_access(handle, inode, path + depth);
-			if (err)
-				goto out;
+		depth = newdepth;
+		ext4_ext_drop_refs(path);
+		path = ext4_ext_find_extent(inode, iblock, path);
+		if (IS_ERR(path)) {
+			err = PTR_ERR(path);
+			goto out;
 		}
+		eh = path[depth].p_hdr;
+		ex = path[depth].p_ext;
+		if (ex2 != &newex)
+			ex2 = ex;
+
+		err = ext4_ext_get_access(handle, inode, path + depth);
+		if (err)
+			goto out;
+
 		allocated = max_blocks;
 
 		/* If extent has less than EXT4_EXT_ZERO_LEN and we are trying
@@ -2452,6 +2456,7 @@ static int ext4_ext_convert_to_initialized(handle_t *handle,
 			ext4_ext_store_pblock(ex, ext_pblock(&orig_ex));
 			ext4_ext_dirty(handle, inode, path + depth);
 			/* zero out the first half */
+			/* blocks available from iblock */
 			return allocated;
 		}
 	}