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().

This was reverted earlier due to a typo in the original commit where I
experimented with using signal_pending() instead of
fatal_signal_pending().  The test was in the wrong place if we were
going to return signal_pending() since we would end up returning
duplicant entries.  See 9f2394c9 for a more detailed explanation.

Added fix as suggested by Linus to check for signal_pending() in
in the filldir() functions.
Reported-by: Benjamin LaHaise <bcrl@kvack.org>
Google-Bug-Id: 27880676
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/compat.c

@@ -936,6 +936,8 @@ static int compat_filldir(struct dir_context *ctx, const char *name, int namlen,
 	}
 	dirent = buf->previous;
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user(offset, &dirent->d_off))
 			goto efault;
 	}
@@ -1020,6 +1022,8 @@ static int compat_filldir64(struct dir_context *ctx, const char *name,
 	dirent = buf->previous;
 
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user_unaligned(offset, &dirent->d_off))
 			goto efault;
 	}

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);

fs/ext4/namei.c

@@ -1107,6 +1107,11 @@ int ext4_htree_fill_tree(struct file *dir_file, __u32 start_hash,
 	}
 
 	while (1) {
+		if (fatal_signal_pending(current)) {
+			err = -ERESTARTSYS;
+			goto errout;
+		}
+		cond_resched();
 		block = dx_get_block(frame->at);
 		ret = htree_dirblock_to_tree(dir_file, dir, block, &hinfo,
 					     start_hash, start_minor_hash);

fs/readdir.c

@@ -169,6 +169,8 @@ static int filldir(struct dir_context *ctx, const char *name, int namlen,
 	}
 	dirent = buf->previous;
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user(offset, &dirent->d_off))
 			goto efault;
 	}
@@ -248,6 +250,8 @@ static int filldir64(struct dir_context *ctx, const char *name, int namlen,
 		return -EINVAL;
 	dirent = buf->previous;
 	if (dirent) {
+		if (signal_pending(current))
+			return -EINTR;
 		if (__put_user(offset, &dirent->d_off))
 			goto efault;
 	}