Re: [PATCH V3 4/5] ext4: get discard out of jbd2 commit kthread contex

["Theodore Ts'o" <tytso@mit.edu>]

On Sat, Jul 24, 2021 at 03:41:23PM +0800, Wang Jianchao wrote:
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 34be2f07449d..a496509e61b7 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3474,6 +3530,14 @@ int ext4_mb_release(struct super_block *sb)
>  	struct kmem_cache *cachep = get_groupinfo_cache(sb->s_blocksize_bits);
>  	int count;
>  
> +	if (test_opt(sb, DISCARD)) {
> +		/*
> +		 * wait the discard work to drain all of ext4_free_data
> +		 */
> +		queue_work(ext4_discard_wq, &sbi->s_discard_work);
> +		flush_work(&sbi->s_discard_work);

I agree with Jan --- it's not clear to me why the call to queue_work()
is needed.  After the flush_work() call returns, if s_discard_work is
still non-empty, there must be something terribly wrong --- are we
missing something?

> @@ -3672,8 +3724,14 @@ int __init ext4_init_mballoc(void)
>  	if (ext4_free_data_cachep == NULL)
>  		goto out_ac_free;
>  
> +	ext4_discard_wq = alloc_workqueue("ext4discard", WQ_UNBOUND, 0);
> +	if (!ext4_discard_wq)
> +		goto out_free_data;
> +


Perhaps we should only allocate the workqueue when it's needed ---
e.g., when a file system is mounted or remounted with "-o discard"?

Then in ext4_exit_malloc(), we only free it if ext4_discard_wq is
non-NULL.

This would save a bit of memory on systems that wouldn't need the ext4
discard work queue.

					- Ted