Re: [PATCH v3 2/2] md: simplify md_seq_ops

[Yu Kuai]

Hi,

在 2024/01/09 7:38, Song Liu 写道:
> On Tue, Sep 26, 2023 at 11:19 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
>
> Just realized this introduced a behavior change:
>
> When there is not md devices, before this patch, we have
>
> [root@eth50-1 ~]# 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
>
> .