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/super.c b/fs/ext4/super.c
index 598498904035..5a97e590692d 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1230,6 +1230,7 @@ enum {
         Opt_inode_readahead_blks, Opt_journal_ioprio,
         Opt_dioread_nolock, Opt_dioread_lock,
         Opt_discard, Opt_nodiscard, Opt_init_itable, Opt_noinit_itable,
+        Opt_max_dir_size_kb,
 };
 
 static const match_table_t tokens = {
@@ -1303,6 +1304,7 @@ static const match_table_t tokens = {
         {Opt_init_itable, "init_itable=%u"},
         {Opt_init_itable, "init_itable"},
         {Opt_noinit_itable, "noinit_itable"},
+        {Opt_max_dir_size_kb, "max_dir_size_kb=%u"},
         {Opt_removed, "check=none"},    /* mount option from ext2/3 */
         {Opt_removed, "nocheck"},       /* mount option from ext2/3 */
         {Opt_removed, "reservation"},   /* mount option from ext2/3 */
@@ -1483,6 +1485,7 @@ static const struct mount_opts {
         {Opt_jqfmt_vfsold, QFMT_VFS_OLD, MOPT_QFMT},
         {Opt_jqfmt_vfsv0, QFMT_VFS_V0, MOPT_QFMT},
         {Opt_jqfmt_vfsv1, QFMT_VFS_V1, MOPT_QFMT},
+        {Opt_max_dir_size_kb, 0, MOPT_GTE0},
         {Opt_err, 0, 0}
 };
 
@@ -1598,6 +1601,8 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
                         if (!args->from)
                                 arg = EXT4_DEF_LI_WAIT_MULT;
                         sbi->s_li_wait_mult = arg;
+                } else if (token == Opt_max_dir_size_kb) {
+                        sbi->s_max_dir_size_kb = arg;
                 } else if (token == Opt_stripe) {
                         sbi->s_stripe = arg;
                 } else if (m->flags & MOPT_DATAJ) {
@@ -1829,6 +1834,8 @@ static int _ext4_show_options(struct seq_file *seq, struct super_block *sb,
         if (nodefs || (test_opt(sb, INIT_INODE_TABLE) &&
                        (sbi->s_li_wait_mult != EXT4_DEF_LI_WAIT_MULT)))
                 SEQ_OPTS_PRINT("init_itable=%u", sbi->s_li_wait_mult);
+        if (nodefs || sbi->s_max_dir_size_kb)
+                SEQ_OPTS_PRINT("max_dir_size_kb=%u", sbi->s_max_dir_size_kb);
 
         ext4_show_quota_options(seq, sb);
         return 0;