udf: Perform preallocation only for regular files

So far we preallocated blocks also for directories but that brings a
problem, when to get rid of preallocated blocks we don't need. So far
we removed them in udf_clear_inode() which has a disadvantage that
1) blocks are unavailable long after writing to a directory finished
   and thus one can get out of space unnecessarily early
2) releasing blocks from udf_clear_inode is problematic because VFS
   does not expect us to redirty inode there and it also slows down
   memory reclaim.

So preallocate blocks only for regular files where we can drop preallocation
in udf_release_file.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/inode.c

@@ -90,19 +90,16 @@ void udf_delete_inode(struct inode *inode)
 }
 
 /*
- * If we are going to release inode from memory, we discard preallocation and
- * truncate last inode extent to proper length. We could use drop_inode() but
- * it's called under inode_lock and thus we cannot mark inode dirty there.  We
- * use clear_inode() but we have to make sure to write inode as it's not written
- * automatically.
+ * If we are going to release inode from memory, we truncate last inode extent
+ * to proper length. We could use drop_inode() but it's called under inode_lock
+ * and thus we cannot mark inode dirty there.  We use clear_inode() but we have
+ * to make sure to write inode as it's not written automatically.
  */
 void udf_clear_inode(struct inode *inode)
 {
 	struct udf_inode_info *iinfo;
 	if (!(inode->i_sb->s_flags & MS_RDONLY)) {
 		lock_kernel();
-		/* Discard preallocation for directories, symlinks, etc. */
-		udf_discard_prealloc(inode);
 		udf_truncate_tail_extent(inode);
 		unlock_kernel();
 		write_inode_now(inode, 0);
@@ -664,7 +661,11 @@ static struct buffer_head *inode_getblk(struct inode *inode, sector_t block,
 	udf_split_extents(inode, &c, offset, newblocknum, laarr, &endnum);
 
 #ifdef UDF_PREALLOCATE
-	/* preallocate blocks */
+	/* We preallocate blocks only for regular files. It also makes sense
+	 * for directories but there's a problem when to drop the
+	 * preallocation. We might use some delayed work for that but I feel
+	 * it's overengineering for a filesystem like UDF. */
+	if (S_ISREG(inode->i_mode))
 		udf_prealloc_extents(inode, c, lastblock, laarr, &endnum);
 #endif