ext4: add max_dir_size_kb mount option

Very large directories can cause significant performance problems, or
perhaps even invoke the OOM killer, if the process is running in a
highly constrained memory environment (whether it is VM's with a small
amount of memory or in a small memory cgroup).

So it is useful, in cloud server/data center environments, to be able
to set a filesystem-wide cap on the maximum size of a directory, to
ensure that directories never get larger than a sane size.  We do this
via a new mount option, max_dir_size_kb.  If there is an attempt to
grow the directory larger than max_dir_size_kb, the system call will
return ENOSPC instead.

Google-Bug-Id: 6863013

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 2a42cc04466..7450ff01c3c 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -55,6 +55,13 @@ static struct buffer_head *ext4_append(handle_t *handle,
 {
 	struct buffer_head *bh;
 
+	if (unlikely(EXT4_SB(inode->i_sb)->s_max_dir_size_kb &&
+		     ((inode->i_size >> 10) >=
+		      EXT4_SB(inode->i_sb)->s_max_dir_size_kb))) {
+		*err = -ENOSPC;
+		return NULL;
+	}
+
 	*block = inode->i_size >> inode->i_sb->s_blocksize_bits;
 
 	bh = ext4_bread(handle, inode, *block, 1, err);