linux-kernel - Re: [PATCH v3 2/2] md: simplify md_seq

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Date: Mon, 8 Jan 2024 15:38:43 -0800
From: Song Liu <song@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: mariusz.tkaczyk@...ux.intel.com, xni@...hat.com, 
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org, yukuai3@...wei.com, 
	yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH v3 2/2] md: simplify md_seq_ops

On Tue, Sep 26, 2023 at 11:19 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> From: Yu Kuai <yukuai3@...wei.com>
>
> Before this patch, the implementation is hacky and hard to understand:
>
> 1) md_seq_start set pos to 1;
> 2) md_seq_show found pos is 1, then print Personalities;
> 3) md_seq_next found pos is 1, then it update pos to the first mddev;
> 4) md_seq_show found pos is not 1 or 2, show mddev;
> 5) md_seq_next found pos is not 1 or 2, update pos to next mddev;
> 6) loop 4-5 until the last mddev, then md_seq_next update pos to 2;
> 7) md_seq_show found pos is 2, then print unused devices;
> 8) md_seq_next found pos is 2, stop;
>
> This patch remove the magic value and use seq_list_start/next/stop()
> directly, and move printing "Personalities" to md_seq_start(),
> "unsed devices" to md_seq_stop():
>
> 1) md_seq_start print Personalities, and then set pos to first mddev;
> 2) md_seq_show show mddev;
> 3) md_seq_next update pos to next mddev;
> 4) loop 2-3 until the last mddev;
> 5) md_seq_stop print unsed devices;
>
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

Just realized this introduced a behavior change:

When there is not md devices, before this patch, we have

[root@...50-1 ~]# cat /proc/mdstat
Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
unused devices: <none>

After this patch, "cat /proc/mdstat" returns nothing. This causes
some confusion for users who want to read "Personalities" line,
for example, the mdadm test suite reads it.

I haven't figured out the best fix yet.

Thanks,
Song