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

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Date: Tue, 9 Jan 2024 09:21:12 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: Song Liu <song@...nel.org>, Yu Kuai <yukuai1@...weicloud.com>
Cc: mariusz.tkaczyk@...ux.intel.com, xni@...hat.com,
 linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com, yangerkun@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v3 2/2] md: simplify md_seq_ops

Hi,

在 2024/01/09 7:38, Song Liu 写道:
> 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.

Yes, that's a problem. And after reviewing seq_read_iter() in detail, I
realize that I also can't use seq_printf() in m->op->start() directly,
because if seq buffer overflowed, md_seq_start() can be called more than
once.

I'll fix these problems soon.

Thanks,
Kuai

> 
> Thanks,
> Song
> 
> .
>