ext4: allocate delayed allocation blocks before rename

When ext4_rename() overwrites an already existing file, call
ext4_alloc_da_blocks() before starting the journal handle which
actually does the rename, instead of doing this afterwards.  This
improves the likelihood that the contents will survive a crash if an
application replaces a file using the sequence:

1)  write replacement contents to foo.new
2)  <omit fsync of foo.new>
3)  rename foo.new to foo

It is still not a guarantee, since ext4_alloc_da_blocks() is *not*
doing a file integrity sync; this means if foo.new is a very large
file, it may not be completely flushed out to disk.

However, for files smaller than a megabyte or so, any dirty pages
should be flushed out before we do the rename operation, and so at the
next journal commit, the CACHE FLUSH command will make sure al of
these pages are safely on the disk platter.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/namei.c

@@ -3005,6 +3005,10 @@ static struct buffer_head *ext4_get_first_dir_block(handle_t *handle,
 /*
  * Anybody can rename anything with this: the permission checks are left to the
  * higher-level routines.
+ *
+ * n.b.  old_{dentry,inode) refers to the source dentry/inode
+ * while new_{dentry,inode) refers to the destination dentry/inode
+ * This comes from rename(const char *oldpath, const char *newpath)
  */
 static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 		       struct inode *new_dir, struct dentry *new_dentry)
@@ -3013,7 +3017,7 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 	struct inode *old_inode, *new_inode;
 	struct buffer_head *old_bh, *new_bh, *dir_bh;
 	struct ext4_dir_entry_2 *old_de, *new_de;
-	int retval, force_da_alloc = 0;
+	int retval;
 	int inlined = 0, new_inlined = 0;
 	struct ext4_dir_entry_2 *parent_de;
 
@@ -3048,6 +3052,8 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 			new_bh = NULL;
 		}
 	}
+	if (new_inode && !test_opt(new_dir->i_sb, NO_AUTO_DA_ALLOC))
+		ext4_alloc_da_blocks(old_inode);
 
 	handle = ext4_journal_start(old_dir, EXT4_HT_DIR,
 		(2 * EXT4_DATA_TRANS_BLOCKS(old_dir->i_sb) +
@@ -3188,8 +3194,6 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 		ext4_mark_inode_dirty(handle, new_inode);
 		if (!new_inode->i_nlink)
 			ext4_orphan_add(handle, new_inode);
-		if (!test_opt(new_dir->i_sb, NO_AUTO_DA_ALLOC))
-			force_da_alloc = 1;
 	}
 	retval = 0;
 
@@ -3199,8 +3203,6 @@ static int ext4_rename(struct inode *old_dir, struct dentry *old_dentry,
 	brelse(new_bh);
 	if (handle)
 		ext4_journal_stop(handle);
-	if (retval == 0 && force_da_alloc)
-		ext4_alloc_da_blocks(old_inode);
 	return retval;
 }