Re: [PATCH] ext4: add max_dir_size_kb mount option

[Jeff Moyer <jmoyer@redhat.com>]

"Theodore Ts'o" <tytso@mit.edu> writes:

> Very large directories can cause significant performance problems,
> especially if jobs are running in memory-tight environment (whether it
> is VM's with a small amount of memory or small memory cgroups).
>
> 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.

I have no idea what a reasonable number for this would be.  Can you
provide guidelines that would help admins understand what factors
influence performance degradation due to directory size?

Finally, I don't pretend to understand how your mount option parsing
routines work, but based on what I see in this patch it looks like the
default will be set to and enforced as 0.  What am I missing?

Cheers,
Jeff

> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
> ---
>  fs/ext4/ext4.h  | 1 +
>  fs/ext4/namei.c | 6 ++++++
>  fs/ext4/super.c | 7 +++++++
>  3 files changed, 14 insertions(+)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index c3411d4..7c0841e 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1243,6 +1243,7 @@ struct ext4_sb_info {
>  	unsigned int s_mb_order2_reqs;
>  	unsigned int s_mb_group_prealloc;
>  	unsigned int s_max_writeback_mb_bump;
> +	unsigned int s_max_dir_size_kb;
>  	/* where last allocation was done - for stream allocation */
>  	unsigned long s_mb_last_group;
>  	unsigned long s_mb_last_start;
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 2a42cc0..bdde668 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -55,6 +55,12 @@ static struct buffer_head *ext4_append(handle_t *handle,
>  {
>  	struct buffer_head *bh;
>  
> +	if (unlikely((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);
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 56bcaec..5896dcb 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;