ext4: allow readdir()'s of large empty directories to be interrupted

If a directory has a large number of empty blocks, iterating over all
of them can take a long time, leading to scheduler warnings and users
getting irritated when they can't kill a process in the middle of one
of these long-running readdir operations.  Fix this by adding checks to
ext4_readdir() and ext4_htree_fill_tree().

Reported-by: Benjamin LaHaise <bcrl@kvack.org>
Google-Bug-Id: 27880676
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/dir.c

@@ -150,6 +150,11 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 	while (ctx->pos < inode->i_size) {
 		struct ext4_map_blocks map;
 
+		if (fatal_signal_pending(current)) {
+			err = -ERESTARTSYS;
+			goto errout;
+		}
+		cond_resched();
 		map.m_lblk = ctx->pos >> EXT4_BLOCK_SIZE_BITS(sb);
 		map.m_len = 1;
 		err = ext4_map_blocks(NULL, inode, &map, 0);