linux-kernel - Re: [PATCH -next] md: simplify md_seq

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Date:   Mon, 11 Sep 2023 16:05:40 +0200
From:   Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     song@...nel.org, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next] md: simplify md_seq_ops

On Mon, 11 Sep 2023 14:50:10 +0800
Yu Kuai <yukuai1@...weicloud.com> wrote:

> From: Yu Kuai <yukuai3@...wei.com>
> 
> Use seq_list_start/next/stop() directly. Move printing "Personalities"
> to md_sep_start() and "unsed devices" to md_seq_stop().
> 
> Cc: Mariusz Tkaczyk <mariusz.tkaczyk@...ux.intel.com>
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  drivers/md/md.c | 124 ++++++++++++------------------------------------
>  1 file changed, 31 insertions(+), 93 deletions(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 0fe7ab6e8ab9..9c1155042335 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -8192,105 +8192,14 @@ static int status_resync(struct seq_file *seq,
> struct mddev *mddev) return 1;
>  }
>  
> -static void *md_seq_start(struct seq_file *seq, loff_t *pos)
> -{
> -	struct list_head *tmp;
> -	loff_t l = *pos;
> -	struct mddev *mddev;
> -
> -	if (l == 0x10000) {
> -		++*pos;
> -		return (void *)2;
> -	}
> -	if (l > 0x10000)
> -		return NULL;
> -	if (!l--)
> -		/* header */
> -		return (void*)1;
> -
> -	spin_lock(&all_mddevs_lock);
> -	list_for_each(tmp,&all_mddevs)
> -		if (!l--) {
> -			mddev = list_entry(tmp, struct mddev, all_mddevs);
> -			if (!mddev_get(mddev))
> -				continue;
> -			spin_unlock(&all_mddevs_lock);
> -			return mddev;
> -		}
> -	spin_unlock(&all_mddevs_lock);
> -	if (!l--)
> -		return (void*)2;/* tail */
> -	return NULL;
> -}
> -
> -static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> -{
> -	struct list_head *tmp;
> -	struct mddev *next_mddev, *mddev = v;
> -	struct mddev *to_put = NULL;
> -
> -	++*pos;
> -	if (v == (void*)2)
> -		return NULL;
> -
> -	spin_lock(&all_mddevs_lock);
> -	if (v == (void*)1) {
> -		tmp = all_mddevs.next;
> -	} else {
> -		to_put = mddev;
> -		tmp = mddev->all_mddevs.next;
> -	}
> -
> -	for (;;) {
> -		if (tmp == &all_mddevs) {
> -			next_mddev = (void*)2;
> -			*pos = 0x10000;
> -			break;
> -		}
> -		next_mddev = list_entry(tmp, struct mddev, all_mddevs);
> -		if (mddev_get(next_mddev))
> -			break;
> -		mddev = next_mddev;
> -		tmp = mddev->all_mddevs.next;
> -	}
> -	spin_unlock(&all_mddevs_lock);
> -
> -	if (to_put)
> -		mddev_put(mddev);
> -	return next_mddev;
> -
> -}
> -
> -static void md_seq_stop(struct seq_file *seq, void *v)
> -{
> -	struct mddev *mddev = v;
> -
> -	if (mddev && v != (void*)1 && v != (void*)2)
> -		mddev_put(mddev);
> -}
> -
>  static int md_seq_show(struct seq_file *seq, void *v)
>  {
> -	struct mddev *mddev = v;
> +	struct mddev *mddev = list_entry(v, struct mddev, all_mddevs);
>  	sector_t sectors;
>  	struct md_rdev *rdev;
>  
> -	if (v == (void*)1) {
> -		struct md_personality *pers;
> -		seq_printf(seq, "Personalities : ");
> -		spin_lock(&pers_lock);
> -		list_for_each_entry(pers, &pers_list, list)
> -			seq_printf(seq, "[%s] ", pers->name);
> -
> -		spin_unlock(&pers_lock);
> -		seq_printf(seq, "\n");
> -		seq->poll_event = atomic_read(&md_event_count);
> -		return 0;
> -	}
> -	if (v == (void*)2) {
> -		status_unused(seq);
> +	if (test_bit(MD_DELETED, &mddev->flags))
>  		return 0;
> -	}
>  
>  	spin_lock(&mddev->lock);
>  	if (mddev->pers || mddev->raid_disks || !list_empty(&mddev->disks)) {
> @@ -8366,6 +8275,35 @@ static int md_seq_show(struct seq_file *seq, void *v)
>  	return 0;
>  }
>  
> +static void *md_seq_start(struct seq_file *seq, loff_t *pos)
> +{
> +	struct md_personality *pers;
> +
> +	seq_puts(seq, "Personalities : ");
> +	spin_lock(&pers_lock);
> +	list_for_each_entry(pers, &pers_list, list)
> +		seq_printf(seq, "[%s] ", pers->name);
> +
> +	spin_unlock(&pers_lock);
> +	seq_puts(seq, "\n");
> +	seq->poll_event = atomic_read(&md_event_count);
> +
> +	spin_lock(&all_mddevs_lock);

I would prefer to increase "active" instead holding lock when enumerating over
the devices. the main reason is that parsing mdstat is implemented in mdadm, so
it could kind of blocker action- for example mdmon follows mdstat so it is read
frequently. The time of getting other actions done can highly increase because
every open or sysfs_read/write requires this lock.

> +
> +	return seq_list_start(&all_mddevs, *pos);
> +}
> +
> +static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +	return seq_list_next(v, &all_mddevs, pos);
> +}
Can it be so simple? Why previous versions takes care of holding "(void)*1" and
"(void)*2" then? Could you elaborate?

> +
> +static void md_seq_stop(struct seq_file *seq, void *v)
> +{
> +	status_unused(seq);
> +	spin_unlock(&all_mddevs_lock);
> +}
> +
>  static const struct seq_operations md_seq_ops = {
>  	.start  = md_seq_start,
>  	.next   = md_seq_next,

Thanks,
Mariusz